As I worked through the assembly of the Kossel ( http://reprap.org/wiki/Kossel ), an OpenBeam based Delta style printer, I decided to build it with dual extruders/hotends and wanted to use the M5 PushFit connectors available from Amazon or TriDPrinting ( http://www.tridprinting.com/Hot-Ends/#3D-Printer-Push-Fit-Fitting )
This part allows the J-Head to snap into place and fits within and will bolt right on the current effector/platform for the Kossel and Rostock.
Updated SCAD file to better fit the inside circle of the Rostock effector.
Update: V2 of SCAD file corrects the misalignment of the single J-head on the insert vs the plate.
The OpenScad file has a variable (printDual) that allows you to specify single or dual hotends. Set this variable and print.
I used PLA with a 0.5mm tip, 0.12mm layer height at 95% infill.
There are 4 captive nuts on each side of the insert which enable you to adjust the height of the hotends on a dual setup. After identifying which, if either, of the hotends is low, I turn the screws on the opposite side counter-clockwise equally. One full turn of M3 bolts is .5mm, you shouldnt need much to get them level.
1 or 2 J-Head MKV
1 or 2 PushFit/Pnuefit connectors
4 M3 X 12mm
2 M3 X 20mm
10 M3 Nuts
6 M3 Washers
I am using this with the modified AirTripper Extruder for PushFit and Planetary Gear Stepper: http://www.thingiverse.com/thing:38069