Hi Moomom,
Your boars are entering pubity essentially. Their relationship is about to be tested and this is a crucial stage in boar pairings.
You will see the following behaviour which is all perfectly normal:
Rumbling and rumble strutting. It's a kind of war dance. It's one of the initial stages in agression/dominance struggles and is a warning as well as a kind of posturing. This is your first signal that something may happen. It's a kind of deep purring noise with your boars moving from right to left leg.
Snorting. Your boars will come face to face with each other and snort from their noses at one another. This is another warning signal.
Circling and spraying. Your boars will each attempt to spray their scent (testosterone) on each other. It looks kind of funny as they will circle each other round and round trying to get a shot in.
Chattering. Chattering their teeth as if they were cold. This is a very aggressive response and is usually your last warning before physical violence.
There are other dominance/aggression signals too. These are all perfectly normal at their age. Male guinea pigs, like any male herd animal, are territorial. When they hit adolescence, their hormones increase and this starts to beomce a problem. It could take up until they are over a year old for this to settle down.
You must keep an eye on them. If they escalate to physical violence and draw blood, you will have to separate them. This is how a lot of boars end up in separate cages. After separation, some baors can be successfully reintroduced and end up living together again but not always.
OUr two were separated at about 5 - 6 months old and have never been able to live together since. They're now happily paired of with younger sows.
I hope this is helpful. I've been through all this myself so if you have any other questions or worries, please feel free to PM me. I'm sure everyone else on here will have good advice too. Good luck and keep us posted
