I think it could be easier to form a pair if both of them are introduced at the same time. That's why I am thing of QT a pair of O.clowns at the same time. But I am afraid that 10 gal is too samll for them. Of course, as Dan mentioned, disease and parasite could problem too. However, I don't want to setup two 10 gal QT at the same time.
Probably I should ask: How to introduce two O.clowns with best chance of paring up?