You could use an ending gcode, I'm not sure how the extruders are distinguished in g-code but you could try something like this which would have both extruders move (and likely leave hanging strings).
G92 E0 0.0 //Sets Extruder 0 position to 0
G92 E1 0.0 //Sets Extruder 1 position to 0
G1 E0 10.0 F1.7 //Moves Extruder 0 10mm at 100mm/min
G1 E1 10.0 F1.7 //Moves Extruder 1 10mm at 100mm/min
I'm not positive right now that is how you call out each extruder, also not sure that the "10.0" correlates to linear movement on the extruder and the feed is 100/60 = 1.6667, that also may be incorrect depending on if feed is mm/s or mm/m.
You could also try
G1 E0 0.0 F1.7
G1 E1 0.0 F1.7
And if the position wasn't reset with a G92 after the retract, they should return to 0.