Delta dual extruders, second extruder slanted yet the first exturder work fine

1.1.0 RC6 without auto bed leveling.
Slicer: S3D

With 2nd extruder is 20mm away toward right side, I set:
#define EXTRUDER_OFFSET_X {0.0, 20.0} // (in mm) for each extruder, offset of the hotend on the X axis
#define EXTRUDER_OFFSET_Y {0.0, 0.0} // (in mm) for each extruder, offset of the hotend on the Y axis
When I print with the 1st extruder ONLY, it works well. If I print with 2nd extruder ONLY, works well.
But when I use BOTH extruder (as object and support), the 2nd extruder print on slanted plane, while the 1st extruder is fine, printing on the bed plane normally.

But when I change
#define EXTRUDER_OFFSET_X {0.0, 0.0} // (in mm) for each extruder, offset of the hotend on the X axis
#define EXTRUDER_OFFSET_Y {0.0, 0.0} // (in mm) for each extruder, offset of the hotend on the Y axis
the slanted issue is gone, although as expected the print from the 1st extruder and the 2nd extruder has offset as their physical nozzle's positions are offset.

My current hypothesis, something wrong related with the EXTRUDER_OFFSET_X and Y.

Is there anyone running the same issue? (sorry if I missed)
Any idea for debug (fixing)?

Edited 1 time(s). Last edit at 07/26/2016 04:47PM by prophet-id.

I haven't played with RC6 version, but AFAIK you better set the offset in marlin to 0,0 and use the offset function in the slicer instead.

I guess the marlin-math for deltas is not ready for offset extruders.

I tried that on my slicer which is Simplify3D, but don't like it since the preview shows each tool (nozzle) set apart (as far as the nozzle's offset) instead as a mono single model.

Not sure about S3D, but with Cura I had to use negative offset values, IIRC...