My cat recently started urinating on the floor. . . .?

The most common (and often only) symptom of a UTI is peeing outside the litter box. It could be stress from you going away. One of the cats could have chased the other one out of the litter box or had a fight while you were gone, but if both cats are spayed or neutered, the most likely cause is medical. My suggestion is to have them both checked for a UTI. If the tests come back ok, the vet will be able to give you suggestions on how to retrain then to start using the box again.