We just added a ocellaris clown to go with the one we've had in the tank for at least a month and they immediately started hanging out together. Only been one full day, and while they're not what I'd call inseperable, they usually don't get too far apart from one another, or for very long. Our older clown Lucy will usually take the lead, with the new clown Desi tagging along. Once in a while they'll be at opposite ends of the tank, but then they'll quickly get back together.
Way too soon to see if they'll form a pair, but so far so good. Desi is a bit smaller than Lucy, but not too much, maybe about 3/8" shorter. My hope is that Desi is small enough for Lucy to become the female of a pairing, but large enough so that she won't pick on him.