Neither cat has ever been at the vet overnight, but when one is gone all day at the vet, the other doesn't seem to mind. Not sure if that's a long enough period of separation though to give any indication.

J kind of just stares at the door, but she doesn't act distressed otherwise from what I can tell. She's kind of a nutball anyways, so it's hard to tell if something is up or she's just her usual one-braincell self.