Don't get a boy and a girl pig. They will mate and you will end up with a mess of babies on your hands and the babies need to be separated from mom at 3 weeks to avoid further mating, plus dad can get mom pregnant immediately after delivery. I would highly recommend two of the same sex, either sex, won't matter! When a pair is introduced at a young age (and most pet store pigs are young) they will bond and do just fine, even two boys.
Please reconsider getting any pet from a pet store. Look for a guinea pig rescue close to you. There are too many unwanted pigs in the world and buying from a pet store only adds to the over population. If you do go to a pet store, make sure you know how to sex them yourself. Don't trust the pet store employees, as most don't have much of a clue.