Atlpax
I'm sure that one of the pros will give you a better answer than this, but here's a starter based on some hours under training in a proper simulator.
The backup ADI is a lot smaller than the primary instrument and is not directly in front of the pilots, so it isn't in their primary scan, which is the area that they trained to focus on. Peripheral vision would not pick up the backup ADI IMHO.
There is normally some type of warning that let's them know if there is a difference between the 2 primary instruments, following which there is a cross check between the two and the back up unit and a decision made as to which is correct.
The pilot with the correctly reading will then take control and fly the aircraft.
As in most things in the airline sector, this procedure is learned and refreshed by practice and safety comes from the swift and sure application by the pilots.
If you take a minute to re-read BEagles posts, you will see his thinking and he speaks with expert authority, which I do not.