If you want to print only one part with printhead 1 or 2. This is always “Single head” mode.
You just have to assign the component (part) to extruder 1 or 2.
And the print configuration is always single head. Not dual!
if you configured 1 part in dual head mode, and assigned this single part to extruder 2, extruder 2 will be effectively ignored, and printhead 1 will be used effectively. A prime tower will also always be printed.
If you want to print 2 parts in separate color, this is possible in Dual Head mode.
eg: assign part 1 to printhead 1
and assign part 2 to printhead 2
Select Dual Head “print configuration” in cura or slicer.
You can also merge parts into 1 effective part.
example a part that consists of 2 or more subfigures.
You can assign part 1 to printhead 1 and part 2 to printhead 2
Select for Print configuration: Dual Head. This I also Dual head config.
This is the “real” dual head extrusion.
example bracelet: and explanation: https://manuals.velleman.eu/article.php?id=25