Is it normal both hot ends are heated when printing via one only? How could it be avoided?
I am slicing on a Cura 3.6.18.
Yes, this is normal because the START GCODE Block for the dual V3 (YellowFin) toolhead assumes that both extruders will be used. Also, during the wiping process for the auto bed leveling, both extruders need to be heated to be cleaned so no plastic residue hinders the continuity for leveling. I was never successful in getting the behavior I wanted using settings in CURA or custom empty materials because even if you create an “empty” material with a print temp of 0C, the START GCODE Block will still try to purge both extruders - which is not a good idea on a cold extruder. This could be mitigated by removing the filament from the second extruder every time, but that is a hassle.
I did get it working the way I wanted by creating a second dual V3 printer profile with modified START and END GCODE blocks that comment out all the T1 related GCODE. I called it “TAZ6 DualExtruder v3 - T0 Only” and use it when i don’t need the second extruder. The result is that the second extruder is completely benign and the result is the same as the single extruder. I keep a text file with all the custom GCODE so that when I upgrade CURA, I can compare updated START and END Blocks from Lulzbot and merge my custom config.