Gordy, I think the tail rotor gear is driven off the #1 side only. I'm looking at a photo I have here of the back of the MGB opened up, and I'm almost certain the tail rotor gear engages the smaller gear just to the front of the red circle you have in the picture.
See pictures below. The diagram from the PTM that you posted seems to imply that the tail rotor gear engages both the #1 and #2 sides but from my photos that isn't possible. The tail rotor gear is straight cut and the ones from the #1 and #2 engine are helical cut.
The pictures show the back of a S76 gearbox. The cover is removed and flipped downwards (i.e. upside down). The tail rotor gear is the big gear on the MGB cover on the lower middle half of the 2nd picture. It engages the smaller, inset gear on the #1 side on the top left. I hope this makes sense!
So yes, if the shaft you circled were to break, the tail rotor would only be driven off the #1 engine!
