They are in an apartment building with rooms throughout that entire hallway to hide in. They wouldn't have to do much to set off the
alarm and then hide in a room near
Burt's armory.
Angel would have to investigate to make sure they weren't robed blind like last time and while he's doing that it allows someone the time to go into the guard room and pop off a few shots and run away. Doesn't automatically mean there are two people working together. Does that mean there aren't two people working together? Nope. I'm just sayin' is all.