What other fish are you keeping n the tank with your angels? Because I have a breeding pair except every time they bred the eggs get eaten and I don't think that unless they are kept in a breeding tank it might not be worth getting another one.
It happend to me, only I thought I had to males, WRONG, I went in their one day and she was laying eggs, i FLIPPED OUT! so unless you whant a bunch of baby angels, I would just not get another male, espessially since its only a 15gall!
I have a 55gal with a breeding pair in and when there are eggs they get 50 the rest get 5g. I just brush them off the uptake pipe and they all have a good snack.