MacPro 6,1 (late 2013) - 4 monitors using only 1 of 2 available graphics cards Late 2013 mac pro 32GB ram, AMD FirePro D500 3072mb. 

All monitors plugged into their own individual thunderbolt ports. 

Performance has been abysmal over the last few months. Many coreaudio errors; screen flickering; mouse jittery. None of the usual remedies have helped (reboot; reset NVRAM et al; re-install). 

Today I happened to notice in the system report that my four monitors (ASUS VS238) are only using one of the two available graphics cards. 

Is there a way to configure the monitors to use a specific graphics card? Does it even make sense to split the 4 monitors between the 2 cards, or is it more sane that it is doing what it is?
I appears it works this way by design: no matter how many monitors are in use, they will be managed by one card, and the other card is in standby for use in uninterrupted GPU computation.