Since your other cat has also started vomitting, I think you've got a G-I bug going. I've had this happen, too. They should get over it in a few days if that's what it is. I don't think you need to rush to an emergency clinic over the weekend unless it takes a turn for the worse or you see something definitely abnormal like blood in the stool. But if it hasn't started improving by Monday, I'd call your vet.
(Actually I think one of the times I had a bug go through my bunch I think it got picked up AT the vet by the cat who was there for something else.)