Separation with boars has to be considered asap after a bloody fight with bites; boars usually don't go back together ever after it.
Incessant mounting and/or bullying to the degree that a piggy is kept away from eating, drinking or sleeping and/or is losing weight is also a reason for separation in both genders. You can usually find out whether a piggy bond is working or not by a trial separation; if the piggy on the receiving end is perking up noticeably when away from his/her companion, then things are clearly not right. However, you can do this only so often as a short term separation can further strain an already stressed bond, so you have to judge things carefully, but it may help to avoid a full-on fight in the case of hormonal boys.
http://www.theguineapigforum.co.uk/threads/faq-introducing-and-re-introducing-guinea-pigs.38562/
With sows, most problems happen during the initial bonding when two dominant piggies cannot solve/accept who comes out on top or if a new member is not accepted. Older, often rather dominant sows are occasionally also the cause that a long standing companion may not be accepted back after an illness or operation. Sometimes, major aggression due to hormonal problems/ovarian cysts can happen, but there are now more less invasive options available than a full spaying operation. Sows can have pretty extreme seasons at times, but things should calm down after a day or two without the bond being broken.
With boars, problems usually happen during the bonding or - in the case of teenagers - during the hormonal months between 4-14 months old. Well bonded adult boars very occasionally get into trouble after a move into new territory when they have to re-establish a new hierarchy or after another change to their lifestyle. If there haven't been any full fights with bites, there is a good chance that they will go back together after a cooling off period.
The general tip for a piggy newbie is to please try not to interfere too much, but to keep a close eye on a tricky situation and ideally to remove any hideys and block any corners with only one access until the crisis is over, as that minimises potential flash points. Re-intros always have to happen on neutral ground.