Can you mix and match SRAM 9 and 10 speed rear mechs? 10 and 11 are fine but 9 and 10? Easier to ask here than 10 both of bikes, swap them and then put them back if they don't work.
What, swapping 9 speed mech for a 10 or vice versa, but keeping same cassette, shifter and chain?
In general you should keep the lot the same as cable pull ratios likely won't be right, but you might get away with it with some index tweaking and having to shift up then down to get the right gear etc, but you might hit problems after things have worn a bit.
Comprehensive answer...
Would say that 9 and 10 seem significantly different enough to not be compatible.
Even 10 and 11, it looks more like you'd get compatibility between SRAM 10sp road/mtb and 11sp road, but not 11sp mtb mechs
10 and 11speed mtb mechs aren't interchangable. Nor are 11 and 12
Yreah pull ratio is the Q. Fair enough then.