The Tosafot (Pesachim 100b, s.v. sh’ein) discuss the practice of covering the challah during Kiddush. There, two reasons are brought: the honor of Shabbat requires that the food not be present, or be exposed if already present, until Kiddush has been recited (Sheiltot D’Rav Achai Gaon, 54, and see alsoRashbam); and the challah should be reminiscent of the “man”, which was covered from above and below with dew. The Talmud Yerushalmi (as quoted by the Tur, O.C. 271), provides another reason, so that the challah not be “embarrassed” at “witnessing” the wine chosen as the preferred item for Kiddush, as challah is also eligible for that purpose. R. Moshe Feinstein (Resp. Iggerot Moshe, O.C. V, 20:18), considers these reasons in addressing the question of a setting where there are many tables, each with sets of lechem mishneh, but one communal Kiddush is being recited apart from all (or almost all) of the individual tables. According to the first two reasons, it would seem that the need to cover the challah would be unaffected. The last reason, however, would be more of a question; it might be argued that as the Kiddush is being recited away from the table, the challah is not embarrassed. Nonetheless, R. Feinstein ultimately rejects this approach, noting that had Kiddush not been recited elsewhere, Kiddush would have to be recited on the challah; the fact that no Kiddush is recited at all at the individual table reflects upon the challah as well. Thus, according to all reasons, the challah in this circumstance should be covered. R. Ehpraim Greenblatt, (Resp. Rivvevot Ephraim, I, 200) comes to this conclusion as well, but brings further support for the possibility that the issue of embarrassment may not attach when the Kiddush is recited elsewhere. However, he counters this with a citation from R. Chaim Kanievsky, noting that since all the people in the room are fulfilling Kiddush, through the mechanism of shomea k’oneh, it is as if Kiddush is being recited at every table. According to all of the above reasons, it would seem that the challah may be exposed immediately after Kiddush. The Maharikh, however, feels that the “man” comparison would require covering until after the berakhah of hamotzi, and that such is the practice, quoting the Prishah that one should then extract the challah from the covering like pulling man out of dew, rather than taking the covering off. The Pri Megadim, though (Mishbetzot Zahav, 271) felt that the comparison to the man is accomplished even if the challah is covered only very briefly, and that the “embarrassment” reason would allow uncovering after the berakhah on wine. The reason of “the honor of Shabbat” would require waiting until after all of Kiddush.