I mixed them together for about a week. The pooping is getting a little worse, my mom said she had the same problem with her dog. I think I'm going to pick up some Natural Balance or Natural Choice today and switch them over gradually. They like the Evo, but I think that's what's making them go to the bathroom more. When I feed them Natural Choice pouches instead there is no runny poop.
It's not the proteins in the food which cause digestive problems, but the grains. Actually, I've heard of some others who had problems with EVO, so maybe there's something in there your cats are sensitive to.
I went ahead and picked up a bag of Natural Balance, which I was planning on using. I bought the Evo because Pet Supplies Plus carried it and it wasn't nearly as expensive as I had seen it online. There isn't any grain in Evo though, so it couldn't have been grain.