Both cats have diarrhea?

Hello, Seeking insight on this matter, Both of my cats have diarrhea the last two days, both male cats, indoor, one is eating and the other is not eating much.  I have not changed their food (they are on wet Royal Canine urinary food, one of them has had severe urinary issues int he past), They are indoor cats, no plants int he house, nothing I can think of that they would have gotten into. I did give each of them very very small bits of salmon we had for dinner the other night, it was very minimally seasons and i even washed it off before I gave then the small bites. I don't normally feed them table food ever.

One of them seems to be using the litter box fine but I did witness him having diarrhea in the box. The other cat is having accidents, several int he last two days, he also urinated outside the box and the mat and then this morning urinated on our couch He is not the one who previously had the urine issues, that was the other cat.

I do have a vet appt. in the morning for them but that was the soonest they could get in. 

Just trying to see if i am missing something or if this may be an issue with this batch of food. 

Thank you in advance for any insight. 

Hard to know. Could be the food. I give extra probiotics and slippery elm when any cats in my house have gastro issues.