(a) The Ritva in Yoma 58a says that it is not a Chatzitzah, but I'll have someone look into it more and get back to you soon.

(b) The Gemara says in Menachos 94a that they must all perform Semichah, one after the other. Otherwise, as you mentioned, it would be a Chatzitzah. (See Rashi there DH Tenufah.)

Mordecai

This is a very interesting question. The following answer is based on the sources which discuss the law of Semichah.

1. Chazal make a statement in Toras Kohanim which supports your assumption that one hand on top of the other constitutes a Chatzitzah. This is cited by the Torah Temimah to Vayikra 1:4. The verse states, "He shall perform Semichah with his hand on the head of the Olah." The Toras Kohanim comments, "His hand should be on the head of the animal and not on the back of his other hand."

This Toras Kohnaim is also cited by the Tosfos Yeshanim in Yoma (36a, DH u'Vilvad). The Gemara there states that when one does Semichah, there must be no Chatzitzah between his hands and between the sacrifice. The Tosfos Yeshanim writes that this Halachah is derived from the Toras Kohanim. If one's own hand is considered a Chatzitzah, then certainly a foreign object is considered a chatzitzah. It is clear that one hand on top of the other is a Chatzitzah for Semichah, and thus there certainly would appear to be a problem with performing Semichah if the head of the animal is too small to fit both hands upon it.

2. However, there is an apparent contradiction between two sources, the resolution to which will provide an answer to your question. The question is, where exactly on the head of the animal should Semichah be performed? The Gemara in Yoma (36a) says that one places his hands between the the two horns on the animal's head. However, the Tosefta in Menachos (10:3) says that one places his two hands on top of the two horns. Where should one place his hands, between the horns or on top of them?

Rabeinu Shimshon of Shantz resolves this problem in his commentary to Toras Kohanim on Vayikra 1:4. He writes that the Gemara in Yoma refers to an animal with a head large enough to place the hands between the horns, while the Tosefta refers to an animal with a smaller head, so that one must place his hands on top of the horns.

3. What, though, should one do if the animal has no horns, or if the animal is so small that there is not enough room to place both of the hands on the horns? A solution may be suggested based on an idea expressed by the Chidushim u'Vi'urim (Rav Chaim Greineman shlit'a of Bnei Brak). In his commentary to Yoma (36a) and Menachos (93b) he writes that it is not necessary to perform Semichah with the entire area of the hands. He adds that there is even basis to argue that it is sufficient if a minority of the palms of the hands are placed on the head. This answers your question.

(See also Ritva to Yoma 58a, DH u'Parkinan, and Kedushas Yom Tov #28, by Maharit Algazi.)

Kol Tuv,

Dovid Bloom

Here is a slight addition to the above answer. If the head of the animal is very small, the recommended approach is to place the majority of one's hand on the head. If the tips of one's fingers protrude outside the area of the skull this does not concern us and it is still considered that Semichah has been performed in a satisfactory way.

KOL TUV

Dovid Bloom