You can add multiple dwarf angels to a mid-sized tank -- but it's generally not a good idea without sufficient planning. You also have to ensure adequate territory for both fish, and that means creating enough living space and feeding territory for the fish. Angelfish are grazers, and jealously guard their feeding spaces from other angels. They'er less aggressive with other fish that they don't perceive to be direct threats, such as butterflyfish, even though they often have similar shapes and coloration.
29g is too small for one angel, let alone two. I have two dwarves in my 65g at home, and I think it's pushing it. They generally do get along, although my multicolor is clearly dominant, and does push away the keyhole at times.