I'm no expert but this is what I've learned or come across on the subject over the years. Some will say it's okay as long as the paints are within the same product category and from the same manufacturer and earmarked specifically for interior. The caveat is that exterior paints generally have additional chemicals or higher concentrations of additivies in them to combat mildew and such and are therefore not appropriate for indoor use. From what I have read, even if you mix the two very, very thoroughly and in a specific ratio that you can reproduce as needed, there's still a chance of inconsistency in the finish where certain spots will have little or no or different sheen when the paint dries. It has something to do with the specific qualities inherent within each paint. That's about all I can contribute to the topic.