I have them separated, Vivi is getting used to the new environment by being in the bathroom, she's mostly hiding but there's not really a lot of spaces in there to hide, but it's like okay we letchu do you
I've already speedrun introducing the cats some by letting them see each other at a distance, while Vivi is in her crate, because of the fact Jiji is generally mellow and Vivi's a kitten, so it shouldn't set off as many "no this is my territory" alarms but that's it
for the litterbox issue, they make these little like, 'travel litterboxes' that are just like, wax-coated cardboard box lids, pretty inexpensive and you should be able to grab a pack at any pet store. so you could set Vivi up with that as her litterbox until you're ready to let them interact unsupervised
which is just gonna reinforce "new cat = bad"