My design of a dual extruder mod for the Lulzbot TAZ. This puts the budaschnozzles as close as possible (40mm center to center). and nearly all of the build platform is accessible to both extruders, once you alter the firmware for extra y travel.
originally discussed here: http://forum.lulzbot.com/viewtopic.php?f=16&t=66
UPDATE 6-25-2013: added a supported version of the motor moved extruder. just cut the support away after you print (thanks to lulzbots design for the help with the support)
print two of the motor moved extruder bodies. all other parts are from the greg wades extruder that lulzbot uses. you need the idler and washer. http://devel.lulzbot.com/TAZ/printed_parts/main_fleet/jonaskuehling_gregs_wade_ao/extruder_idler-washer/ and of course the heringbone gears. http://devel.lulzbot.com/TAZ/printed_parts/main_fleet/herringbone_large_gear/ http://devel.lulzbot.com/TAZ/printed_parts/west_fleet/herringbone_small_gear/
It's a little tough to get the motor screws tightened with the gear in place, but it can be done. you will also have to cut the excess threads from the hobbed bolt, and likely remove the thumb screw caps and one of the springs.
print the extruder mount and mount the 2 assembled extruders and budaschnozzles. wires for the nozzles should come up from between the extruders.
print the adjustment block, probably will need support material. Install M4 nuts in the sockets in the adjustment block.
now that you are done printing the parts, remove the extruder mount.
Remove the 4 screws on the bottom of the x carriage. slide the adjustment block up the x carriage until the screw holes line up, you will likely need to file some parts to get it to fit but it should be tight. replace the 4 M3 screws with longer ones to bolt everything together. add 2 M4 bolts or set screws pointing up. they will push against the dual extruder mount to raise the second nozzle to get them both at the same level.
wire everything up, and upgrade the firmware configuration.h and configuration_adv.h .
don't forget another PTFE or HDPE feed tube. And I still need a good dual spool holder.
ALSO- I strongly suggest upgrading the 3 linear bearings in the X carriage to ball bearings. the excess weight of the extra extruder will accelerate the wear of the solid bearings.