Vanessa,
A second basenji is super easy compared to one. They entertain each other, have the same interests, and temperment.
But, if you are looking for another breed, here are a few breeds that Caesar absolutely loves to play with at the dog park:
Labs
Australian Shepards
Here are the breeds that Caesar has had trouble with overall at the park:
German Shepards - something about them makes Caesar scared
Pitts - treat my basenji like prey and are a little too hard when mouthing play
Brittanys- freeze hunt and dont play when the basenjis want too
I think that the play drive will be important. I would look for a dog that will retrieve or run to keep your basenji's interest. Caesar loves to chase the other dogs, but isnt interested in the same goal.
Jeepjeep is trying to find a home for Rowdy on this thread:
http://basenjiforums.com/showthread.php?t=1766
He seems to be a really sweet basenji and his temperment is very cute.