How can I get my two guinea pigs to stop fighting?

I've had two male guinea pigs for about 3 years now. They are both approximately the same age, one is slightly younger, maybe by about six months or so. We bought the older one first but realized he was getting a bit lonely, so we went and got another baby male. For a while they got along great, they would cuddle up together at night and play together in their grazing ark. But when the younger piggy reached one year old, they started fighting aggressively and blood was shed. We bought separate hutches and they've been like that for two years now, would it be possible to re-introduce them to one another? If so, what would be the best way to do this? They're both looking very sad and lonely, it would be great if they could live together again. I'm willing to do whatever it takes.