I have an idea: maybe conceal yourself and spy on them until one or both set down their guns to pick up some valuable, then blast away!
In this situation, I have to agree with the majority to position yourself in the best concealment so you can view the stairs, your first shot will be aimed and surprised. Hopefully your next shots will be true. You can probably squeeze of a few shots before the second BG can react and hopefully you can hit both BGs.
Does anyone have a problem with shooting them both in the back if the opportunity arose; say they both walked directly past you and didn't see you and were walking into an open room with their backs to you. Now is the perfect opportunity to put one in each of their backs at 10' range. Do you shoot?