About a year ago I rebuilt my MakerGear M2 as a dual semi-independent extruder configuration which I called the "twin extruder" (I had previously redesigned the M2 to run off of a Smoothieboard). Since the configuration had both extruders connected to the same X belt but allowed them to be placed at any fixed distance from each other, the setup was a natural for "ditto printing", or the ability to run more than one print at the same time by forcing one or more secondary extruders to respond to all of the commands sent to the primary extruder.
Because the RepRap definition includes "M605 S2" for duplicating the movements of one extruder by another, and because this command was previously unhandled in Smoothieware, I made use of it for ditto printing. "M605 S2" causes all non-selected extruders to follow the selected extruder. "M605 S0" returns to normal behavior. I have a video of my M2 running in ditto printing mode at opencircuitdesign.com at ~tim/3d_printer/twin_project/ (as a "low karma" user, apparently I'm not allowed to post links, so you'll have to put that one together yourself). The ditto printing video is at the bottom of the page.
My fork is on github at RTimothyEdwards/Smoothieware and the branch is "feature/ditto-printing". There is a link to it from the Smoothieware wiki page for 3rd-party forks. Hopefully somebody will find it useful. Questions, comments, and suggestions are welcome.