Hello Gordy,
I remember having seen a different main gear box internal design from this one (wich is the "current one") where it was easier to understand this case of MGB failure;
1/ eng1 gearing connects MGB and T/R
2/ eng2 gears MGB only.
So MGB is geared to T/R via ENG1 and its free wheel. In case of eng1 power shaft failure to the MGB, NR will drop, eng 2 will take over the loss of power and in precipitation,you may believe you have an engine 1failure (if you don't look the N2) ... you may be tempted to shut eng1 off, and if you do so, you will loose your T/R ...
But I also know that at least 2 models of gear boxes have been manufactured; the 09500 and the 09600, to cope with the increased power of the PT 6 of the B model (and later for 2S Arriel series)
Could it be that the MGB internal drawings that we see now on training documents is this one of the 09600 series that would include this gearing modification ? Difficult to tell, but it can be compared with the drawings found in older training manuals of A versions
In this case this unlikelytricky scenario would apply to A, A+ and A++ only.
Note: No worry if you don't understand what I'm trying to mean, it is normal, it's not you ! English is not my mother tongue )))