Not used a 2x11 but it's trickier with a 2x10 than a 3x9 as the cage is narrower.
There's no obvious need for it, but there you go! I'd imagine that an 11x is narrower again, but haven't looked at/seen then to compare in person. I've used a 2x10 XT with 2x9 and it just needs careful alignment. Given the prices of used 9x stuff why buy new!?
If you look around carefully you can pick one up for £2.99
