Can you mix flat interior latex and semi gloss interior latex?

Yes, but the finish will be somewhere around satin. You can only do this if the paints are of the same type, i.e. either both latex, or both enamel (alkyd). Make sure you mix them together very well, and continue to agitate while you're painting.

Post edit: They may mix and go on alright, but interior and exterior paints are, however, formulated to have different properties. Exterior paint actually may not hold up as well indoors (Bob Vila). Most importantly, though, chemicals used in exterior paints outgas and are harmful for indoor use. Try some of the new low-VOC interior paints. They are much better for your health.