Originally Posted by
Sy Reene
If Di2 allows it, it must be ok (once in a while).
When the electronic shift systems were first coming out I attended a seminar given by 1 of the manufacturers on setting up their system. One of the attendees asked why the system wasn't smart enough to prohibit the use of forbidden combinations like the big-big or small-small. The instructor laughed and said simply that there are no forbidden combinations. When pressed he said that there were actually other combinations that had higher losses and/or wore the drivetrain worse than the big-big or small-small, but they saw less than a 1 watt and less than 1% difference in wear from the best to the worst combinations. So the simple answer was to use them all.
Funny thing is sometime later they came out with a software update that would prevent the big-big and small-small from being used. Must have been due to everyone "knowing" that these combinations shouldn't be used.