you need cover as in plants, driftwood etc..the angel that is getting picked on can take refuge.
angels are very difficult to sex..unless they are spawning there is no hard fast rule that works.
I find that the males tend to be a bit bigger than the females.
in the case of my spawning pairs they are noticably bigger..however, that is not set in stone as in one pair the female is as big as the male.
I do notice the domestic angel males tend to have a more pronounced bulge on their heads(hump) but in my wild angels this is not the case..so as you see..you may have fish of the same sex..
I actually was able to get lucky and guess the sex of mine..when I had a femae in a pair die..I added what I thought was two females..turns out they were both females as they are both now paired up with mates and spawning.. a 40 is large enough to house the angels..
make sure you have enough cover for the fighting pair to retreat.