females that used to get along

Question
We have two females, one who is almost 4 years old and one who is almost 2 years old.  They have always been in the same pen and always shared food and treats.  Recently, they even slept nuzzled together.  Three weeks ago we discovered the older (and dominant) one had a uterine mass and cystic ovary.  She was spayed and has recovered.  After the surgery for one week we left up a divider in their pen until she healed.  At first everything seemed back to usual, until I heard a fight.  We have since left the divider up and tried "reintroducing" them on neutral ground.  I have also tried switching their sides of the pen after changing the bedding and giving each of them the other one's bed.  It appears the the dominant one is being the aggressor in the attacks and the submissive one is still trying to be submissive.  Do you have any suggestions?  Thank you for your time.

Answer
almost all questions i am having at the moment are regarding pigs falling out, i wonder if there is a season when they are more prone too?

fear is the best tool when the divider does not work, the principal idea is to scare both of them while in the same place (eg a box or carrycase). instinctively they huddle together because they feel sasfer, this "we're safer together" mentality then starts to stick. so things like a bath, car journey, being shuck (gently) in a box. one of my personal favorites if to place them in a gardesn run with a stool chair and let the dogs have a run on the lawn, the dogs dont really bother with the pigs but do play quite rough growling and bounding about the place and the pigs cant really hide away from them so get scared and huddle.

just use the principal of fear and then leave them in a neutral ground together and see if they get along, if not keep trying.

best of luck