Could you post your config?
I am curious how it looks like with 2x512MB Z2 cards 
Configured as Z3, not Z2.
Drop down menu under memory, scroll to #1 Z3 Fast memory and move the slider to your desired amount.
Drop down menu under memory, scroll to #2 Z3 Fast memory and move the slider.
I believe most are just choosing 1 block but Toni suggested using 1 512MB and 1 256MB which I initially did but then wanted to test moving it to (2) 512MB.