"Paired" clowns generally refers to one of two things. It could mean that the clowns are a proven breeding pair, exhibiting egg-laying behavior. Or, it could mean that the clowns are possibly a breeding pair (living together and acting like a potential breeding pair). Basically, it's a way of an lfs saying, at the very least, "we're pretty sure this is 1 male and 1 female, and they both live together without killing one another over territory.
Getting a set of "paired" clowns can be a good thing, though. A "paired" set of clowns is able to live together, more likely to be easy to distinguish the male from the female, and, generally, older. Why is older better? Because clownfish do take some time from that tiny little baby guy at Petco to grow into a nice, mature adult. So, if you're really interested in breeding or seeing mating behavior in a hurry, it can be a nice thing.
Otherwise, you can always find your own pair by starting with a small group and just narrowing it down to two.