1. I don't think there would be a problem, as you are describing the question. The only thing you may notice is a change in the texture and consistency between one color and the next. To avoid any potential reaction, make sure the paint dries thoroughly before adding the next color.
2. You didn't ask, but I can see the follow-on coming: I definitely wouldn't mix the two to get a particular color shade. Try to use paints from the same manufacturer for that process.
Gip Winecoff