Mike,
I don't know whether it is specifically accepted or not. As far as I can see, pulling the tape twice and getting two readings would constitute two measurements. But, because of a possible tendency to read the same thing wrong, when doing it a few seconds apart, I felt that using the metric/imperial scales would be a good check. The mind does not instantly convert the distance, so there is no lingering knowledge of a "correct" reading - thus the two measurements are independent.
At least that's the way I see it.
Your camera idea is a good check. I don't use a spring scale.