This is essentially the same design as my popular Replicator 2 Mk8 extruder upgrade http://www.thingiverse.com/thing:42250 its just been adjusted to fit the geometry of the Replicator 2X - so the instructions are basically the same. You will just need to use one of the long countersunk 3M screws from the 2X extruder to mount the arm, you also re-use the bearings, all you need is a countersunk M3 x 10mm and a countersunk M4 x 15 for each extruder.
UPDATE V2 (April 28th 2013)
I have added a filament feed tube connector to the top of the arm, and a few other requested refinements.
Print, install and enjoy!
I have uploaded Lefthand (L) and Righthand versions.