Nope, that is the only option, especially with this being a 20 gallon tank. I have a pair of clarkiis in a 75 and I would never chance adding another clown to that tank, it would be dead with in days.
Mixing different species of clowns is generally a bad idea, with bad outcomes. (( there are a few exceptions, but they are very few, and when you factor in tank size, it will not work in this case ))