I've been thinking about this for months, and working at it for a few days. Last weekend I got a dual hotend carrier printed and converted to a Bowden system. Last night I did some design tweaks and printed the last couple of items I needed for the 2nd extruder, and salvaged the extruder and hotend from my other printer (an i2 which I haven't used for months).
The little ghost purple square to the left is because it was dribbling out of extruder 0 while drawing with extruder 1 on the first few layers.
The Marlin firmware extruder offset thing doesn't seem to work - or I don't have something set right. It didn't add the offset to the movements. So I just had Slic3r do the adjustment, I'll figure it out later.