Can't remember if I ever did it with both covered simultaneously, but I distinctly remember having to demonstrate several circuits with each of the altimeter and the ASI covered.
However, I learned something of relevance a couple of weekends ago whilst doing my tailwheel checkout on a decathlon. This particular machine was devoid of many avionics, including no stall warner. My accomplice for the training took us up to 4000ft and we did some stalls and made me take note of the stick position. It's an extension of the trim principle, and quite obvious when you think about it. Stick position sets airspeed. In unbanked, unaccelerated (i.e. 1G) flight, if you don't change the configuration (flaps, load etc), the aircraft will stall at a constant stick position. If the stick's forward of that position, it won't stall. End of story.
Now a decathlon doesn't have flaps - so we made a lot of approaches in some pretty leery sideslips where the ASI doesn't necessarily read quite right. The whole stick position thing worked rather nicely.
Just another tool..