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.
sNApple is right I should have been more blunt with that recommendation to go to dendroboard.com.
Our local zoo does have a display with some kind of Dendrobates (black with greeen stripes?) and some big tree frog, but the whole thing is 5-feet tall or so and the tree frogs hang out at the top near the lights while the darts live way down at the bottom.
In general dart frogs should not be mixed with anything. In very large enclosures, some can get away with mixing different species of dart frogs, but even that is frowned upon and will result in possibl aggression/release of different toxins or even hybridization. Go with two seperate enclosures if you want darts and red eyed tree frogs. If you can't do that just go with one.