(continued from above...)
2) I devised a way to do this on a practical level if you can't get the hang of the first method.
This is a ruler just to show a continous flow of counting:
1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7
7s in the RH and 6s in the LH to start with. So because we are tapping 6s against 7s/7s against 6s, we mark the RH (7s) and count to 6 and mark on 7 (every 6 numbers), and for the LH 6s we count to 7 and mark on 1 as shown below (every 7numbers). L and R indicate the hand you tap with at that point in time on the ruler:
1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2
7s R R R R R R R
6s L L L L L L
I think this is slow because at first you will be counting and it will slow you down. You can devise the same for other polyrythms.
I devised this and wondered whether it was logically correct. Then I saw this site on 3s and 4s and look to be similar and was pretty please with myself because Im not at all mathematical even though Im an IT consultant/programmer

as a second career to music of course
https://en.wikipedia.org/wiki/Polyrhythmplease let me know if this helps!!