Yep. Mixing frogs like this is pretty much a no-no to most. It can probably be done, but I would really recommend against it.
However if your vivarium is pretty large, the frogs may never even meet each other. Dart frogs can be pretty adventurous, but the RETs I've kept generally aren't, but that doesn't mean they aren't. At night, mine would perch somewhere that gave them a good view of the tank and scan for food. Then jump down, grab a bite, and head back up to the perch.
The size is the main problem. A small dart frog could be a snack for a large RET. Dart frogs are usually more expensive than RETs, and who knows what kind of damage a dart frog would cause to an RET while being eaten. Their toxins are mostly gone, but it could still at least harm one, possibly both frogs.
Last edited by wondabread; 10-22-2009 at 10:02 PM.