This is the same thing I was wondering. I have a single boar, Pucca, who is about 1.5 years old. Does he really need a friend? I'll post a link to the cage he's in. I don't think it has room for two gpigs and I can't afford a new cage. By the way, I didn't buy this cage, it came with him as I rescued him. Is it to small?
All Living Things Deluxe Guinea Pig Starter Kit - Cages, Habitats & Hutches - Small Pet - PetSmart