The opinion of the Rambam [Eidus 20-2] is that the פטור of "כאשר זמם ולא כאשר עשה" - the עדים זוממים are only punished when they ATTEMPTED to impose a punishment but not when the punishment was actually carried out, only applies to נפשות, capital cases. But when it comes to זממה דמלקות - when the falsely accused was given מלקות, there is no פטור and the עדים זוממין receive מלקות. The Raavad sharply disagrees. 

The Chasam Sofer [חו"מ סי' ל"ג] explains the opinion of the Rambam based on the Magid Mishna [פ"ב ממאכלות אסורות ה"א] who wrote that in a place where there is an explicit לאו הבא מכלל עשה [an aveirah derived from a positively framed mitzva], we may now derive punishment from a קל וחומר [even though normally the rule is "אין עונשין מן הדין" - punishment may not be meted out on the basis of a קל וחומר]. That is what the Rambam means: Even though לא תענה is a לאו שאין בו מעשה and therefore doesn't mandate מלקות, nevertheless, once we already have "לא תענה" we can then learn from a קל וחומר in order to give מלקות, just like a לאו הבא מכלל עשה allows us to learn a punishment from a קל וחומר. The קל וחומר goes as follows: If there is מלקות when the עדים merely ATTEMPTED to have the person punished, most certainly there is מלקות when the עדים actually brought about מלקות in practice. And like we said, the קל וחומר cannot be rejected by saying אין עונשין מן הדין, because there is a לאו of לא תענה [forbidding false testimony] that allows us to go forward and derive a punishment from a קל וחומר. However, when it comes to דיני נפשות - capital punishment, the rule of the Magid Mishna doesn't apply [why not? I don't know! E.E.]. So now we understand why the Rambam says that even if מלקות were already given, the עדים זוממין receive מלקות, but the same doesn't apply to דיני נפשות.  In the words of the Chasam Sofer: 

"והנלע"ד בכוונת הרמב"ם עפ"י מ"ש ה"ה פ"ב ממאכלות אסורות בדעת הרמב"ם דהיכא דאיכא לאו הבא מכלל עשה ואיכא נמי ק"ו עונשין על אותו הדין ופשוט דכל מיני לאוין דאין לוקין אי איכא ק"ו עונשין וא"כ הכא איכא לא תענה אלא דהוה לאו שאין בו מעשה ועוד דאיכא לאו הבא מכלל עשה דמדבר שקר תרחק כמבואר ר"פ העדות וא"כ עונשין מן הדין דהשתא ומה כאשר זמם לוקין מכ"ש אם עשו אשר זממו וכקושית אבוה דבריבי וזה שייך במלקות אבל במיתה אין ממיתין מן הדין ע"י לאו הבא מכלל עשה וזה פשוט ומבואר והי' זה שלום לך ולתורתך כנפשך היפה ונפש א"נ דש"ת. משהק"ס מפפד"מ". 

Based on what we wrote, we may question this explanation of the Chasam Sofer. The M"M was talking about a case where both the קל וחומר and the עשה related to the identical איסור. In such a case the M"M was מחדש that it is enough that there is an איסור עשה on order to make it possible to learn punishment further from a קל וחומר. But with respect to עדים זוממין, the לאו and the קל וחומר are not relating to the same איסור. The קל וחומר relates to the punishment of כאשר זמם that is not coming as a punishment for the איסור but rather המשכת החיוב - the drawing of the obligation of the falsely accused on to the accuser [as we explained]. While the לאו relates to the עבירה of false testimony ["לא תענה"]. In such an instance we cannot combine the קל וחומר and איסור to give a punishment and we return to the rule of "אין עונשין מן הדין" - We cannot punish based on a קל וחומר.

However for this very reason the opinion of the Rambam [that we give מלקות to the עדים זוממין even after the accused received מלקות and we don't say "כאשר זמם - ולא כאשר עשה"] makes perfect sense and we can't ask on him that "אין עונשין מן הדין" [which would be the only way to give the מלקות, as we explained - If they get מלקות in a case where the accused had not yet received מלקות, certainly in a case where he did]. Since he holds that the punishment of an עד זומם is unique and anomalous among all other punishments insofar as that it is not merely punishment for evil acts but המשכת החיוב, he can also maintain that the general rule of "אין עונשין מן הדין" doesn't apply to it. 

What emerges from what we said is that the punishment of מלקות of עדים זוממין and doesn't require the normal conditions for punishments but rather it is "המשכת החיוב" -  the drawing of the obligation of the falsely accused on to the accuser. 

According to this it would appear that we can offer another explanation [different from what we said earlier in the name of the Shaar Hamelech] for Tosfos that we mentioned earlier who says that when עדים זוממין testified about someone they falsely accused of being מוציא שם רע on his wife and wanted to make him liable to מלקות and ממון, they too are liable to both מלקות and ממון since "רצו לחייבו שניהם" - they wanted to make him liable to both [even though normally the rule is אין לוקין ומשלמין - we don't give two punishments]. What they mean is that the very basis and foundation for the punishment of the עדים זוממין is what and how they wanted to make the accused liable, therefore we can't say אין לוקין ומשלמין. That rule applies to regular punishments where we are guided by the principle "משום רשעה אחת אתה מחייבו ואי אתה מחייבו משום שתי רשעויות" - for one act of evil we obligate him and not two, so only one punishment allowed. But when it comes to מוציא שם רע where the accused would have received two punishments, that liability is transferred to the עדים זוממין, who then automatically will receive the dual punishment.   

According to this explanation we can understand the Raavad who said that if someone steals and eats  חלב, he has to pay כפל, and we are no longer troubled by the question we asked, that it is עדות שאי אתה יכול להזימה [because they would have to receive a dual punishment, for the stealing and the eating and since we can't mete out two punishments this עדות is not "hazama-ble"] and thus invalid עדות. For according to our analysis, in a case where they plotted to obligate the accused in two punishments - they would also receive the duo. The punishment of the עדים זוממין is an anomalous case where we don't consider it like a classical scenario of "משום רשעה אחת אתה מחייבו ואי אתה מחייבו משום שתי רשעויות" because it is not about the "רשעה" - sin, but rather המשכת החיוב of the accused. Since he would get both - the עדים do too. Therefore, this עדות can be "hazama-sized" and is thus kosher!! 

Rebbi Akiva Eiger [דרוש וחידוש] asked on Tosfos from the gemara in Sanhedrin [9a] that if the father of the girl who was accused by her husband's witnesses of unfaithfulness, brings witnesses who made the עדים into עדים זוממין, the law is that they are killed but need not pay. This is despite the fact that they plotted to obligate the woman both capital punishment and the monetary loss of her Ksuba [some people prefer to say "Ketuba" and that is cool too!!]. But according to Tosfos who said that when the עדים attempted to make the accused liable to two punishments they also receive two punishments, in this case the עדים should also get מיתה and have to pay - a double whammy. Why don't they?? 

Answered R' A"E that when they cause the woman to lose her Ksuba that is not considerd a punishment but rather since she was unfaithful, the obligation of the husband to pay the Ksuba is automatically erased. Not a punishment but a consequence. So it turns out that the עדים didn't attempt to make the woman liable to two רשעיות [punishments for one sin]. That is different than the case Tosfos discusses of מוציא שם רע when they wanted to obligate the husband in two punishments [מלקות and ממון]. In that case they in fact receive both punishments.   

We see from R' A"E that he understood Tosfos according to the first explanation we offered, that since they schemed to obligate the husband in a monetary penalty that is not exempted by קים ליה בדרבה מיניה, they also receive that liability and there is no קלב"מ for them either. And that is what he answered by saying that the woman is not liable to two punishments, so the עדים aren't obligated in the monetary liability. The rule of זממה is that it obligates in punishments and not automatic consequences. 

However, based on our understanding that is not what Tosfos meant at all but rather that מלקות of הזמה is not given as a punishment but rather as המשכת החיוב - the drawing of the obligation of the falsely accused on to the accuser and therefore these מלקות don't have the status of "רשעה" [punishment] that would exempt them from the monetary payment. 

But for that very reason R' A"E's question is not difficult because this principle we established relates only to מלקות where we can say that it is not a punishment but מיתה always has the status of "תורת דיני נפשות" and always exempts the lesser punishment. This is because permitting Jewish blood to be spilled is always defined as דיני נפשות and we can't call it המשכת החיוב [because of its severity?]. If so, whenever the blood of a Jew is permitted to be spilled it has the status of רשעה and thus exempts a lesser punishment. 

The proof is that a רודף is פטור from ממון even though he does not receive the punishment of מיתת בית דין but rather anyone can kill him. It must be that the very fact that he must die is what exempts him because any permission to spill Jewish blood has the status of דיני נפשות. We also see this in the Yerushalmi that a בן סורר ומורה who is sentenced to death for his thievery is exempted from the monetary payment. Now the בן סורר ומורה is not killed as a punishment but "על שם סופו" - because of the sinful future that we envision for him, so why is he פטור from ממון? It must be that a Jew can't "סתם" have a דין that he must be killed but rather it must be subsumed under the more formal חיוב מיתה or דיני נפשות and thus has the status of a punishment which would exempt him from monetary payment [note - Rav Chaim Halevi disagrees. See the end of the 6th perek of Geirushin]. Due to this, we cannot say that עדים זוממין are both killed and pay because מיתה always creates a מצב of קים ליה בדרבה מיניה and is always considered "דיני נפשות". So we can understand why the witnesses of the husband are killed and don't pay, as the gemara says. 

[עפ"י תורת הגאון רבי חיים שמואלביץ זצ"ל]