Why a single guinea pig is a lonely guinea pig
Pick the bonded pair. Guinea pigs are herd animals from the ground up, and in several countries keeping one alone is actually discouraged in law because the welfare problem is so well recognised. A lone guinea pig has nobody to call to, follow, or settle beside, and owners often mistake the result - long stillness, hiding at every sound, little of the bright wheeking a contented pig makes at feeding time - for a calm temperament. It is usually a subdued one. Human company helps, and daily lap time matters, but you cannot be in the pen at dawn when guinea pigs are active, and you cannot speak the quick vocal language pigs use with each other all day. A companion is not an upgrade for this species; it is the baseline.
Why bonded beats buying two strangers
The word bonded is doing real work in this question. Two guinea pigs who already know and accept each other - littermates, a mother and daughter, or a rescue-matched pair tested side by side - move in as a functioning unit. That spares you the introduction process, which for adult males especially can be tense: unfamiliar boars may posture, chatter teeth, and occasionally fight seriously enough to need permanent separation, leaving you running two pens. A rescue that pairs pigs for you has already done that matching work and will tell you honestly which pig is bolder, which hangs back, and how the pair behaves at feeding time. Same-sex pairs are the standard route; a neutered male with a female is the other combination rescues commonly place.
Space and setup for two pigs
A pair needs genuine floor space, because guinea pigs are ground dwellers who exercise by looping and wandering their pen rather than climbing. Think in terms of a long, single-level pen you could almost lie alongside, lined with fleece or paper bedding, with two of everything that matters: two hideouts with separate doorways so a pig can always leave an argument, two water bottles, and hay available without limit in more than one spot. Feed a staple guinea pig diet with daily vitamin C through fresh vegetables such as bell pepper, because guinea pigs, like people, cannot make their own. Set the pen where household life passes by - guinea pigs are social with people too and do best where they can watch the room - but out of direct sun and away from radiators.
Where to find your pair
Start with a guinea pig rescue rather than a shop shelf. Rescues hold the bonded pairs this decision calls for, know each pig's age and health history, and will often let you meet the pair and handle both pigs before committing. Ask what diet the pair is on, whether either pig has had dental or skin issues, and what support the rescue offers in the first weeks. A good local exotics vet is the other essential contact, since guinea pigs need a clinician comfortable with small herbivores. To line up both before adoption day, search KinsLe Pet Services Directory for guinea pig rescues, sitters, and small-mammal vets in your area and connect with them while your pen is still empty.