The meowing when she pees is not that uncommon. None of mine do it, but I've heard others here who's bengals do. Bengals are loud, talkative cats in general, and some just "monologue" a lot, and let you know that they are having a pee or poo or whatever
Definitely mention it to the vet though next time you go there (like Sherry said).
It's your call whether to keep them together or separate them. If they were adults, I'd definitely keep them apart, especially if they are hissing. But kittens? I dunno. Is either showing any signs of being fearful of the other? Kittens like to go through all the things that cats do (halloween cat arching of back, hissing, growling, puffing up fur and tail etc) and many times they are just "practicing to become cats", not even meaning any of it in the slightest... just goofing off. But if there is actual fear involved, then you might separate them, just for a short time. If you do separate them though, I'd give them frequently re-introduction opportunities, and "cheat" by giving them treats when you bring them back together (to build up positive associations). I'd be really surprised if they don't take to each other very quickly.