When you say they have bitten bits of the others ear - has this left lasting damage? are we talking a lot of biting or the occasional nip? Is it being left bloodied and scabby?
In my experience a bit of ear nipping is generally expected, my own Lily has a tiny little scab on the very outer of one of her ears probably from Daisy but this has stemmed from the death of the dominant piggy and them two trying to sort out who is now going to be boss.
There are a couple of guides on boars that you might find useful. I have never had a boar pair so my first reaction is to read these so that you can notice when the bond might be unsuitable and the boars may need separating:
Boars: A guide to successful companionship.
Boars: Bullying, Fighting, Fall-outs And What Next?