Can you provide a bit more information please
How big is their cage and do all hides have two exits?
How old are both the piggies?
How long have they been together?
What exactly is going on between them?
Bullying is a specific set of behaviours which extend beyond dominance and go into one piggy not being allowed to eat and therefore loses weight, becomes withdrawn, constantly being chased out of hides and never allowed to rest, constant mounting and chasing.
If is what is occurring then the bonds in trouble guide below will help you with how to spot it and what to do next. A separation can be used to decipher if their bond is functioning. If you separate and the bullied piggy perks up, looks happier, moves around more, then that is a sign of a dysfunctional bond and a permanent separation is probably needed. The bully will always be upset by the separation so it isn’t his reaction you go by.
Bonds In Trouble
Boars: Teenage, Bullying, Fighting, Fall-outs And What Next?