Although the main purpose of adding multiple extruder feature to the RepRap machines is to allow printing the dissoluble support material, the new RepRap X2V3 dual carriage design opens a few new interesting possibilities.
The machine supports driving each extruder hotend independently on the X axis (”Dual X Drive”, see video here http://www.youtube.com/watch?v=AEW-DNo0GxI). Its predecessor (RepRap X2) had both hotends fixed on the same carriage (similar to Makerbot’s dual extruder machines) and at relatively close distance of about 20mm. If printing simultaneously from both extruders the machine’s build volume on the X axis would be limited to the distance between the extruders (not quite useful). Because of that I’ve decided not to implement the “follow-me” mode while adding multiple extruder support to Marlin firmware.
Unlike it’s predecessor RepRap X2V3 is free to position the hotends at any distance the size of the machine allows. That makes it possible to effectively use all the build space (i.e. the build envelope for simultaneous printing on RepRap X2V3 is about 100×200x130mm). Moreover, independent carriage driving allows simultaneous printing of mirrored copy of the object. That is often required when printing parts for the dual extruding machine or any object that has mirror symmetry. These hardware capabilities have convinced me to add the “follow-me” support to the latest (v1.1.0 X2 Beta 1 as of this writing) version of MarlinX2 firmware. The command for turning it on/off is M322. Its description is available in the README.md file. The video below demonstrates the feature in action: