I wouldn't have them in anything less than a 40g tank. Preferably with at least 1 of the clowns hosting in a nem. I 2nd the idea of using an acclimation box, I would have the smaller one sit in the box for at least 1 week before fully introducing. There is a high chance for aggression, and so it is best to have a LOT of hiding places so the clowns can have their own space if anything were to arise.
You want the larger one in the acclimation box not the smaller one. The smaller one will be more inclined to share the tank than the larger one which will attack any clown coming in.
Make an acclimation box out of Tupperware and put the larger one in there for three days. Observe them interact and only release him when aggression has subsided. You'll probably see one of them twitching (submitting) which would be a great sign.
I did this in a 90 gallon and there was zero aggression after I released the bigger Maroon
Ok. Yeah that's what I did starting last fri. First I separated them, and notice the little somehow squeezed through the hole, he seem fine with the bigger clown, and I did not see any aggression but I notice the little one submitting and staying in one place. I will observe again today