Jrf bro... just so you have the terms correct for future use:
one solid pushrod is called a push-pull setup.
A pull-pull setup can have cable or solid push rods.

Also your reference to the fact that one cable is "expected to go slack" on a correct pull-pull setup is again totally incorrect. Any reference to the word "slack" used in an aerobatic context should be seriously reconsidered as this will only create problems.

Most of us spend a great amount of energy eliminating "slack" at all cost, be it incorrect pushrods, clevis's, servo gears, hinges etc etc. I suggest you should try doing the same...it make quite a difference.