I came back to SolidWorks after a while and stuck.
I split fixed length into two cuts d1 and d2.
d1 + d2 = fixed
make d1 driven
make d2 = d1
expected: d1 = d2 = fixed / 2
(It works this way in Inventor)
yes, here constructional geometry with 2 equal cuts solve the problem.
Thank you so much for any response!