Enough to say that its not recommended. Especially in smaller tanks, and ESPECIALLY with the 2 species that you are thinking of. Fire clowns are the spawns of satin IME. Probably not as bad when they arent pared, but I gave away a good breeding pair because they were the most evil things I have ever seen. I do have a single fire clown that I raised from her parents, and she isnt as nasty, but I still wouldnt pair her with another species. Maroons can also be evil.
All this beingh said, there is a small chance that things would work out fine for you, and I am sure that there is someone that will chime in saying that they have done it. Odds are they wont tell you that they have only been in the hobby for 3 months, and that there clowns hjave only been together for 5 days, and that they have a lion fish that is keeping both clowns scared enough that they aren't bothering each other..
Bottom line.. Not recommended!