Originally Posted by
fooferdoggie
my other bike has the long cage and had this 11-42 cassette on it and the same 46 chain ring. I thought about just changing the RD's around on the bikes and be done with it. the tandem only has a single chain ring the 46.
The swap might work - or might not, depending on the tandem's geometry. I understand that "pushing the envelope" re: a RD's max sprocket is often dependent on the frame.
Presumably both RDs are Shimano DynaSys 10-speed. Since you're running the tandem 1x and have both RDs and the cassette on hand I don't see how it could hurt to try. You can always back out the changes if the new config won't work or doesn't work as well as you need.
Depending on your other bike's front config, the swap of RDs might possibly cause problems there. Shimano says the GS only has a total capacity of 35 (see my addition to my comment above about specified RD-M786 capacities), and an 11-36 cassette takes 25 of it. That doesn't leave much left for front difference. But you might get lucky there, too.
Best of luck, and hope everything works out.