Depending on some other behaviors, this could be them courting. Does the male make a real queit buzzing sound? When he makes this sound the sides of his stomach will press in and out. You might also be able to notice if the middle toe on the back feet are tapping real fast (like noticed when frogs eat). If all these things are going on, chances are good that they may breed. My female strokes my males back alot around the coconut huts they are going to lay eggs in. Otherwise it looks like she is riding him around the tank for short distances. Hopefully this is what is going on, have you gotten any eggs from them before? Is this your first experience with breeding (hopefully) dart frogs? Good luck with them!!!