If they are brought up together from quite an early age they should be okay. I had a lone male age 5 months and I got another of the same age and they were fine. They can also be okay if they are neutered but if older unneutered males are put together they will usually fight.
Hi I have kept G pigs for many years and providing they have got plenty of space will live together happily. They are social animals in their native land and live in groups.
Neutered males with no females will generally get on OK, so long as they have been social with other GP's from an early age. Entire males will always fight and more so if the is a female around.