Why Two (or More) Guinea Pigs are Better than One

It seems that when most people decide to get a guinea pig as a pet for the first time, they decide to get one, thinking that one is "all they can handle." The thing is, guinea pigs are social rodents who enjoy the company of a buddy, especially if you're going to be out of the house a lot. I find that many people get one guinea pig, only to decide much later on that they would like another, but by that time, adding another guinea pig might lead to fighting or injury if one has been used to living alone.
It is best to keep guinea pigs in groups of 2-5, in a cage that is large enough for them to run around in freely. It is possible to place two unrelated pigs together as adults, but it needs to be a careful process. They must be introduced on neutral ground, not simply thrown together in a cage, even if the cage is clean. Females are more likely to get along than males, but even pairs that have been together their whole lives will still "fuss" (tooth chatter, butt wiggle, hump, chase, etc.). This happens with both males and females and is perfectly normal; as long as no one is getting hurt, it's fine.
When pairs or groups of guinea pigs arrive at our rescue, we keep them together, and try to adopt them out together. I realize that this results in fewer adoptions, since again, most people want only one, but I will not separate pairs or groups that have always been together, as this can be traumatic. On the flip side, I will not force two unrelated, same-sex, single guinea pigs to live together as cagemates. I have far more failures than successes in this arena. If, after the second or third introduction, the guinea pigs aren't getting along splendidly, I will not put them together. Every guinea pig has a different personality, and if one is more dominant, it will only lead to problems if you make introductions recklessly.
So, if you are considering adopting, it would be best to adopt a pair, even if it means a little more cage-cleaning, etc. In the long run, it will be much better for your pet.