Added a leadscrew to the Z-axis and tested the alignment of different tools.
Looks promising, as long you start excactly at the center of the bed.
With two pens at different origins, the initial position becomes even more critical.
I need a better way to adjust the xy position of the pen on the Z-axis to calibrate the start position,
but better do that directly when installing the hotends for a first print.
One big error, I only noticed when adding the gear around the Z-axis in my last post, was that I didn't compensate for the additional rotation (around itself) of the bed while it is orbiting around the Z-axis/sungear.
In the first version the sungear was so small that the slight distortion wasn't that visible.
It became more obvious when increasing the size of the sungear for fitting the leadscrew and rods.