“Uter” refers to one of two. “Uterque” is both of the two. For the difference between “uterque” and “ambo,” you’d get the best answer by consulting a dictionary. How much is what I say worth?
This note is from the Lewis & Short entry for “uterque.”
The difference between ambo and uterque is thus given by Charis. p. 49 P.: Ambo non est dicendum, nisi de his, qui uno tempore quid faciunt, utpote reges Eteocles et Polynices ambo perierunt quasi unā; Romulus autem et Africanus non ambo triumphārunt, sed uterque; quia diverso tempore). http://perseus.uchicago.edu/cgi-bin/philologic/getobject.pl?c.0:2178.lewisandshort