This is up to now, the latest version for Marlin Firmware for GEEETECH Prusa I3 Dual Extruder. Works great!!!
I edited it with Geeetech parameters for prusa I3 Dual Extruder.
Compile with latest arduino 1.8.2
Do not forget to setup your extruder distance in configuration.h file, like below ->
#define HOTEND_OFFSET_X {0.0, 32.25} // (in mm) for each extruder, offset of the hotend on the X axis (BIGGER means 2nd extr go to left)
#define HOTEND_OFFSET_Y {0.0, 0.00} // (in mm) for each extruder, offset of the hotend on the Y axis
With this setting you don't need to setup any offset in slicer. Extruder separation is given from firmware.
Also it is necessary to setup an offset distance in order to print in center of the bed->
#define MANUAL_X_HOME_POS -17.0
#define MANUAL_Y_HOME_POS 0
