I know that most people say that you can't keep two together, but I have had an Eibli angel and a Keyhole angel in my 55 gallon for about a month and a half now (my wife picked up the Keyhole without me).
I had heard that one way to help ensure that they will leave each other alone was to rearrange all of the rock in the tank to eliminate established territories (we have had the Eibli for about 3 or 4 months). I moved all 70lbs of live rock around and put them into completely different areas of the tank. For the first day, I thought I would have to take one or the other out, as they were chasing each other. I actually tried to catch the Eibli to give him a "time out" in the QT, but was unable to (it was late and I just gave up
). By the next evening, the two were co-existing rather well. Now, they are best buddies (I think that they went to sensitivity training or something).
I agree with everyone else, it is a hit and miss, but if you are willing to do some moving around your tank (and a willingness to get one or the other out of the tank if things turn ugly), then it is possible to have success.