If you buy them in a pair that would be fine, I don't see a problem with it. It is hard to buy one, then to try to add another at later date. They can be aggresive. In my opionion they are the best kind of clowns to have. I have a Maroon clown, with a little Maroon gold stripe. matter of fact they laid eggs a week ago.
Hope this helps
yes, a pair is fine, i would watch them to make sure that they are compatible b4 taking home, jsut incase they have already paired up, the only potential problem could be what else is in the tank
HTH
my clarki and volitan do fine together. and they are in a smaller tank. a yellow tang is a great mate for a clarki. these two make me laugh all the time.