as with most things in this hobby, you are hearing what MOST people find to be true. In general, dwarf angels (or larger angels) do not like each other. Depending on the size of the aquarium, amount of LR and hiding spaces, when the fish are added, and just plain luck...you may or may not have success with mixing them.
I had success personally in my 55 with an eibli and a keyhole, though this was done by accident (my wife placed an order from SWF when we already had the eibli and she fell in love with the keyhole...oops!!!). When the keyhole was ready to go into the DT, I had to rearrange all of my LR to get rid of the territory that had been claimed by my eibli. I still thought that I was going to have to get one out, but they calmed down after a day or two.
I would not advise mixing them in anything smaller than a 6 foot tank....the more room that they have, the less they will see each other, which will mean the less likely they are to show aggression.
Your best bet if you want to do this is to add them at the same time (assuming that your tank is large enough).