NCAA rules game:
2&G on B-5 line. A40 runs to B-2 where he losses possesion on a fumble. Ball rolls into endzone, where it is recovered by B55. B55 tries to advance, but still in the endzone fumbles. Ball is recovered simutaneously by B70 and A50 in the endzone.
Touchback?

Touchback is correct. B's ball 1st and 10 at the B-20. The ball was put into the end zone by A's impetus. Simultaneous catch or recovery of a fumble is awarded to the team last in possesion.

Same for NFHS. A good example of "force" and possession.

It was the force of A40's fumble that placed the ball into the end zone. Team possession changed when B55 obtained the ball. B55's fumble left the ball in team possession and the simultaneous possession leaves the ball in possession of team B as a touchback.

Canadian Ruling
 

Touchdown.

HAHAHAAHAH just kidding.

B 1D/10 at their 20.