For a dual-extruder setup on an i3v, I keep thinking the easiest route would be to print out a second set of X-carriage parts, and get 3 slightly longer bolts to go through the wheels to attach a second carriage to the back of the original carriage. This would sandwich the extrusion between the carriages, which would share the wheels. You'd lose 0 X distance, but maybe too much Y distance. Though any lost Y distance could be compensated for by using longer Y axis extrusions and belt.