CNC Machine displacement error

Do I guess correct that your axis with the green "belt" is the one that gives you an error?
If you do not measure the real surface, I'm confinced that you will never get it precise.
One effect/reason is called trum ( in German... No chance to translate to English as word always replaced by a.....) other are thickness error, slipping, tention changes...

The only way would be some kind of roller, but that will have a slight slop or you go the marking way. Means you identify position on belt by either magnetic or optic strip.

I did a lot in that Metier when industrialized the digitran digital printing process. ( nope, not Kammann; they only bought the rest after we got broke)
Mike

Circumference of a round is 2RPi or DPi i.e.
you have a round shape that is 100mm in diameter, multiply that by Pi or 3.1415 and you get the distance of the surface of that round, so 314.15mm
You need to do some math there to have the spin at the same speed.
Or do you intentionally need them to be separated and at different speeds ? I can not think of any idea why.

i believe you made a great point,
the motor spin speed should always be the same for both the upper and the bottom layer.

That's probably why I'm having a error when ploting arc/circles using G02, G03 commands. I set the bottom layer Y axis, and the upper layer B axis, probably, G02/G03 only coordinates Y axis with X axis, thus B axis moves independently.

So, if there is a simple way to make G02 commands treat B-axis the same as Y-axis.