I've recently got a rainbow dragon. I bred cactus and blue fire dragons. How do you breed a double rainbow dragon?
Taken from the Dragonvale wikia:
The Double Rainbow Dragon can be bred using any two dragons, in either order, that contain at least four different elements at the Breeding Cave/Epic Breeding Island.
The trick to doing this is making sure all the elements of the dragons that are breeding differ from each other.
A good example is River dragon and Sonic dragon. This combination has a chance to produce a double rainbow dragon, as well as a Olympus dragon.
Also from personal experience, I've received a double rainbow dragon from breeding a Salamander dragon and a Scoria dragon.
This will most likely take some tries, as it's a pretty rare dragon!