it is summarized here (according to the translator's very limited understanding) http://dafyomireview.com/398#sum where it says "the third premise states that anything composite cannot be eternal..."
(I happen to be the translator BTW)
Basically what he is saying is that only that which is truly infinite can be eternal and if there were two gods, then neither one would be truly infinite since each one would be bounded in some way hence neither could be eternal.