I know someone who keeps two male capra with two female coronata... And you're talking to him!
I have been keeping the males together since I got them, and they get along just fine. And the two females I have had for a while now.
"of course, they would have to be of the same species..."
Obvioudly that isn't true. Mine don't even pay attention to each other (at least not in a that way)
I do use a 6'x3'x2' vivarium so They all have their space. That might be an issue if you use something too small.
And I would be worried if you were planning to keep more aggresive dragons/species together. All my MHDs are super tame and get along with me and with themselves. Another possible concern.
I think FroggieB has kept two species together for a short while once. I believe she said they didn't show any courtship behaviour so I don't know if that is really a possibility.
One of those females is gravid right now, so she might have cross-bred. Or the other female could actually be a male. I'm still in the process of determining her sex. Or, which I am putting my money on, that she is so content she is laying eggs just for the hell of it.
But, I don't think they will cross-breed. And there you go. I think it would be fine mixing to species of Acanthosaura as long as you leave yourself a healthy alternative.
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Ryan