As a rule, Dwarf Angels do NOT like their own kind...It is possible that they will fight each other to the death...ACBlinky has been lucky to have had success with that many in a small tank..I won't recommend placing two in the same tank unless its a 6' tank . They need to look completely different and need to be added at the same time..
If you want to place these in your 55, I'd say its a bad idea..tank is too small for these fish who tend to have very LARGE attitudes