The only advice with a pair of Clowns is 'hope they work out'. Doesn't matter if they are the same species or otherwise, they will go through the ritual, and there is ALWAYS a chance that one of the two will die. That's just how it is with Clowns. If it works, it works. If it doesn't, it doesn't. If you are to keep Clowns as pairs, prepare to have one die. Because the line between acceptance and killing is very thin, and before the dominant one makes its decision, the two will go through fights.
Okay sure there are exceptions, and sure some species are just more aggressive than others, but once it comes to pairing, it's all about chance. I had a Maroon and a Skunk together fine, likely comepletely and utterly. I've also had a Clarkii completely go all out on a Occy when it was pairing with a Tomato.
I've seen that same Clarkii get bullied by its Clarkii mate at the LFS, and the Occy was the dominant of its Occy pair at the same place.
So there.