I have three McCosker's in my quarantine tank right now. I just finished acclimating them. I had to go to two different stores to get a trio. Two of them are juveniles (I think/hope) and the third is a very small male. He is already flashing the girls. Since there are no threatening fish in the QT, they are swimming freely and even had a snack just a few minutes after introduction to the tank. When QT is finished, I will be offering the odd male up for sale in the local reef club. I'm sure somebody will be happy to have him.
Sassy, I think the McCosker's and Carpenters are so similar in appearance that it would make little sense to have both. Choose one or the other. At least around here, the McCoskers are more readily available. LiveAquaria occasionally offers three McCoskers as a package deal; two females and a male. That would save you the effort of identifying the gender and hoping that the juveniles you find are still female.