belt is common and useful item (coupling) between motors and ball screw reducing the dynamics and revolution with effect of the inertia -I saw it in the Hurco VMC as well that a had worked on earlier.
Short belt is accurate:long synchronizing belt -I do not know . The only thing is sure - producer gives 0,1-0,2 mm accuracy on a belt driven linear unit on 1 m long.(so the elongation is the accuracy)

You are right double ball screw means double inertia and its effect to the required power fourfold.
That is why I am here to overcome this type of doubt , so thank you to share your experience