If anyone is thing of copying this, I made an additional change which you might want to consider:
This dual-x carriage design requires that the distance between the two endstops is properly calibrated to ensure that the printing from the two extruders is aligned. I found that that the variability in triggering of the mechanical microswitches led to inconsistent alignment of the two materials (the X and Y endstops in a normal printer don't need to be very precise). I solved this by replacing the microswitch endstops with Hall-effect endstops ( http://reprap.org/wiki/Hall-%CE%98http://reprap.org/wiki/Hall-%C... ). Search ebay for 'Hall Endstops' - they are around $10 each. I used bigger trigger magnets than the ones supplied but that was simply for ease of positioning.
I also replaced the Z-axis endstop with a Hall-effect endstop which is completely unrelated to dual x-carriage support but gives a nice improvement in first layer height consistency. [ I'm sure I could design a better Z-axis endstop holder but some Polymorph/Plastimake/InstaMorph/Polycaprolactone (https://en.wikipedia.org/wiki/Polycaprolactonehttps://en.wikipedia.org/wiki/... - great stuff - get some if you don't have some, the uses are endless) and a cable tie is working fine for the moment. ] The endstop already includes a trimpot for fine adjustment of the trigger point.