What types of anemones do you currently have?
I'd recommend just one pair of clownfish per tank. Especially with Clarkii clownfish.....they're very aggressive. They'd probably end up killing your other pair of clownfish.
I'd give your current pair some more time, and see if they'll end up using your anemones as their host. My bf's pair of Ocellaris clowns took a good 2 months or so before they decided to use our RTBA as a host. Now, they won't leave it. They "discovered" the anemone shortly after we started putting pictures on the tank that showed clownfish in anemones. Not sure if it actually worked or not, but you could always give it a shot.
And just for your information, there is no all-black Percula clownfish. If he/she is all black, you most likely have an Ocellaris.
Good luck!