Hi Gordy
Having read Nick's reply, it is consistent with what I intended when I described the photos. Both engines normally drive the Main Gearbox and therefore the Tail Rotor. The Tail Rotor drive gear is *meshed* only to the left hand, #1 side. However the left hand (#1) side is also indirectly driven by the #2 engine through all the other gears in the MGB.
The diagram you posted from the FlightSafety PTM with the red circle is drawn ambiguously, and seems to suggest that the Tail Rotor gear is intermeshed simultaneously with the #1 and #2 engine input gears, which is not how it actually is built if you look at the photos.
So yes, if the shaft you circled were to fail, I would agree that the Tail Rotor would be disconnected from the Main Gearbox and #2 Engine, and the #1 Engine would be the only thing powering the Tail Rotor.
I hope I haven't added to the confusion!