Keep a towel and/or thick gloves handy
If the guinea pigs attack one another, it is dangerous to put your bare hands in to break them up. Their teeth are extremely sharp and your hands may be injured in the frenzy. Keep a towel and/or a pair of thick gloves to hand while attempting an introduction so that you are prepared should you need to split them up.
Mask their personal smells
A great way I discovered to mask the scent of both guinea pigs (especially boars) is to apply a little bit of "Vicks VapoRub" on the nose just above the nostrils. The smell is so strong they can't actually smell eachother, so hopefully they won't feel like there is a territorial contest. You can get "Vicks VapoRub" from your local pharmacy.
Introduce in a large space
When pairing guinea pigs it's advisable to do it in a large area where they can get away from eachother. A large space means they won't feel trapped in eachother's company and avoids putting them under unnecessary stress.
Stick to your guns
Boars generally take longer to accept one another than sows. With most pairing attempts of both genders, there is a lot of mounting, chasing, teeth chattering and squaring off with open mouths. As long as there is no obvious physical attacking with intent to harm, then your guinea pigs have a chance of living together.
Sometimes it's just not meant to be, and some guinea pigs just don't like eachother. You must end the introductions if the following happens:
They turn to face eachother with open mouths and noses in the air, and the chattering becomes very loud and fast (this is an angry sound)
The guinea pigs physically attack eachother with intent to harm.
Repeat
If all is going well, then repeat this method of introducion several times before you place them into a home together. You can usually tell when it's time to try them in the same hutch or cage because they get to a point where they're not fussed about being in eachother's company and they do very little. They may even be sharing vegetables by this time and happy to give eachother a good sniff.
Shampoo them and create neutral territory
Finally, give them both a bath in the same shampoo to give them the same smell and place them in their new home with clean bedding, bowls and bottles to start them off with fresh and neutral territory. They should establish their own pecking order and hopefully their companionship should be peaceful.
There is no foolproof method of getting two guinea pigs to live together, and sometimes it doesn't matter how much effort you put in and how well they get along for weeks, months or years down the line, one day they may decide they just don't want to be friends anymore.