You can't sex young angels. This is why they are kept in larger groups - usualy of 6 or 8 - and allowed to grow out. once they've sexualy matured, pairs begin to form. The remaining fish are returned/rehomed and a pair kept. Being a compatible pair that formed of its own accord, these two fish get along. The only conflict arises if they start breeding or if the tank's too small - at which point intruders get chased away, or even attacked.
And that's the next point - they are cichlids - all be it relatively peaceful ones. Like most cichlids, they are territorial and become aggressive during breeding. A tank with lots of heavy planting or rockwork etc - lots of ornaments basically - always helps keep individuals from fighting by breaking up the tank into distinct areas each fish can call its 'territory'.
You also need to appreciate that a larger angel is bound to pick on a smaller one and, therefore, they need to be the same size. Similarly, and established fish, like yours which has been in its tank a while, is going to attack any newcomer as an intruder.
Also, you mentioned you'd kept pairs, but also kept them in threes. When angels are kept in threes, one is going to be the lowest ranking and will constantly be harassed by both the other two. Furthermore, if 2 pair up, the odd one out will be chased and bullied - usualy to death.
I appreciate you say you only want 2 but, if you are prepaired to do so, it would be best to return/rehome the one you have currently and get a group of 6 young ones. once paired, remove the extra 4 and keep one pair. With lone angels, you also run the risk of it suddenly turning aggressive as single angels do tend to do.
Your other option is to try to find another angel of the same size, or a little larger (but only very slightly) than your own. Re-arange the tank before adding it and then just hope things work out
