The "rule" as I have read it is 1 dwarf angel not 2. As Adam stated, many have had different experiences (including myself). The larger the tank the better.
You'll have better luck if you introduce them at the same time. If you can't and it's FOWLR, then rearrange the rock work a little.
Whenever I add a new fish I always keep my lights off all day until I am ready to add and then I add the fish while the others are sleeping. Fish often find a place to hide right away and this way they won't claim someone elses house as their own since the owner will be home sleeping. I dont know if my theory works for that reason, but it seems to work for me.