That is also frowned upon. First of all most darts are very territorial. Second, you don't want tobe producing hybrids or mutts. Darts breed easier than most frogs. Even keeping two or morphs of the same species is frowned upon. If you kept, say, cobalt and citronella tincs together any offspring produced would not be welcomed in the dart frog community and you would be stuck with them.

Just one more thing red-eyes get big enough to eat most darts.