in a 55g i wouldnt keep 2 of ANY angels. Although to be honest, i have and would keep a yellow tang with that group in that size tank. Although there will be trouble down the line, yellows dont seem to grow too awfully fast, and you can often get small 1-2" ones that are healthy. Another good tang choice could be a scopas or the mimic tangs, both of which are relatively hardy, small, and peaceful as tangs go.
Jon