It can depend on the natures of the pigs as well as the conditions, sometimes you can do everything you possibly can to avoid any altercations, and they still decide they cannot stand each other all of a sudden!
The important thing is to not panic if you see them communicating in certain ways which may seem aggressive, as a lot of people think they are fighting when chasing and mounting for example, which is natural boar behaviour and quite normal unless it's constant and goes on non-stop in an established pair for more than a day or so. Our Pepe and Leo are happily bonded but will have little phases where one will hassle the other for a while and if they are getting fed up, we just seperate their run with a mesh divider for just a few hours and this has always calmed them down.
A lot of the gossip about boys not being able to live with each other is mainly down to humans not understanding their pigs' behaviour and mistaking normal dominance behaviour for fighting.
If you are worried about a particular type of behaviour, just type a thread on here or go through the existing ones in the Behaviour board and we will all do our best to help!