I've been lucky up to this point with boys who were already bonded and stayed that way, but our current boys have had a much rockier road. To read the full story you can check out the thread "The Adventures of Popchop and Fuzzable" in Guinea Pig Chat, but briefly: we got them when Fuzzable was about 6 months old and Popchop about 4 months. They fell out dramatically after a few weeks when Fuzzable bit Popchop's ear in half, so then they lived in a divided cage.
A lot of people are happy with boars living side by side, but we didn't think it was ideal when they were so young and there were other options. So we decided to neuter them, on the basis that it would give a small chance of calming them down enough to rebond them, and if that didn't work, then they could each be found a girlfriend.
In fact, we won the jackpot and we did manage to rebond them. As per all the advice, we set up a big pen with freshly washed bedding, no hideys or bowls, just a pile of hay and water bottles.
They started in very dramatic, lots of rumbling and loud squeaking, displaying by walking crabwise towards each other. But it only lasted about 10 minutes before the little guy either realised that Fuzzable was bigger, or decided he didn't want to be boss pig after all, and backed down! It was fascinating to see, very clear and unambiguous. Fuzzable is, in the end, not an aggressive pig, and took his victory very gracefully. After another hour or two of Popchop being very nervous and Fuzzable expressing that he wasn't going to attack, they ended up sleeping, not together, but about a foot apart, with Fuzzable having chosen the sleeping place and Popchop following him.
Ever since they've been very happy together. We do get rumbling and occasional noisy arguments, but they both seem very content with their positions and I don't foresee any problems in future.
If you want lots more boar bonding stories, check out Scotty's Animals on YouTube. He's got a whole playlist. Interestingly he doesn't follow the neutral space rule, but he's a seasoned expert pig whisperer, and I think the rest of us should stick to those rules to give as much chance as possible for bonding to work out.