Pulisa is correct, but be prepared for lots of chasing around and mounting each other. This is normal and shouldn't be mistaken for aggression.

Signs of aggression: teeth chattering, "squaring" up to each other, raised hackles, teeth baring, lunging at each other. This is extremely unlikely with a baby, I've never seen an adult attack a baby in over 45 years of keeping piggies, but if it does happen at a later date never, ever, try to separate them by attempting to pick them up because you will get a very nasty bite. Instead you should try to part them with something like a dustpan or rolled up newspaper and leave them to calm down separately for about half an hour before handling them again.

Personally I'm not a fan of neutering unless absolutely necessary (testicular cancer for instance, or when every attempt to paint them with a same sex piggy has failed) because they don't deal with anaesthesia and surgery at all well, and to be perfectly honest there aren't many vets who I would trust to operate on my pigs, they just don't have the experience.