Doug wrote:
For any backup to be effective, it must be in the scan at all times.

Indeed. I flew with somebody recently in his own airplane. Very
nicely equipped, with a 2nd (electric) AI mounted just to the right of
the altimeter, so it should have been in his scan. I was also blessed
with a whole array of pullable circuit breakers in front of me, which
I decided to take advantage of :-)

At one point, I pulled the breaker on the AI. I saw the red flag
immediately pop out, but he didn't notice it for about 10 minutes.
Later in the flight, I covered up his DG (HSI, really) and we did some
partial panel work. What I didn't tell him was that I had also pulled
the breaker on the TC (again, with a instant red flag on the
instrument face). He didn't notice that either.