what age and sex are they , are they housed together, if so then they will have to be kept seperate once they have had a serious fight otherwise one dead and the other possibly serious injuries, did you get them both from pups, .
once a bullbreed decides he doesnt like something its a devil of a job to change his mind ,they will usually bide their time.
more exercise together might help as this will re-enforce the pack bond.
all types of shock collars are banned in wales i think . the only ones you are allowed are the citrus spray or sonic ones , these will not stop a determind dog fighter.