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Pippy
Would adding the four middle pulleys double the torque and resolution ?

I'll vote for "not really".
It seems like a reduction only for horizontal axis - when both motors spinning at same direction.
When motors are spinning as cw/ccw - I can't see any reduction in vertical axis, because a whole block system is unused.

It is just a brief look, I can be wrong)

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Pippy
That's a lovely machine prot0typ1cal !

Do you have a video of it working I wonder ?

Old video, nothing recent:
CoreXYU