I would say it actually depends on quite a few things, like the size of your tank and the size of your existing angelfish. By reading your signature, I'm going to assume that your lone angel is in a lightly stocked 70 gallon tank. If this is correct, you should be fairly safe. Is your angel fully grown? Do you know if it's a male or a female? Angels can be very territorial, particularly if they've spawned before (because they'll be wanting to spawn again). I would try to get an angel that's as close in size to the one you've got as possible. In a 70 gallon tank, I don't think you'll have too many troubles, but there can be difficulty if you add a sexually mature male to a tank that already contains a sexually mature female.