1. Introduce the boars in a neutral territory outside the cage.
2. House them in a big enough cage (with enough food, water, toys, etc for two pigs).
3. I have found that giving boars a bath can calm aggression and neutralize a bad situation (especially if they are spraying each other).
4. Also, thoroughly clean the cage and all toys and houses with a vinegar and water solution. This can help make boars less territorial.
5. My boars spar occasionally when I accidentally put a toy or house in their cage that smells like my girl skinny pigs. Keep girl scents away!
6. My friend has two male guinea pigs who got along a lot better after the dominant pig was fixed. But remember, any surgery has risks!
If possible, the pairing should include a more submissive boar and a more dominant boar. The best pairing is two baby boars, or an adult pig and a baby. Pepper was introduced to Pickles as soon as he was weaned. Pepper has always been a very submissive pig, which is one of the reasons I picked him to be Pickles' cage mate. They get along great (even better than the two girls!) and are constantly cuddling and eating side by side. Not all male pigs will get along, but there are definitely steps you can take to increase your chances of having a happy boar pair!
Even if your boar cannot live with another boar, guinea pigs are social animals and still appreciate having another guinea pig in the room! Solo guinea pigs are sad guinea pigs.
Here are some great links on introducing new new pigs and boar compatibility: