I know they are both neutered Helen and I am sure someone much more experienced in this than me will be able to help, but the only thing I can think of is to to go back to basics, remove them both into some neutral territory and let them sort out the dominance. Sounds like Piper wants to be a real Little Miss Bossy Boots. Will be interesting to see how they behave here on neutral ground shortly, I'll naturally keep a close eye on them for you.