Well atm (at least until dx12) video card ram doesn't work that way. Video card ram acts as 4+4=4 instead of 4+4=8, so 2 titain zs would still just be 12 gb of ram.
At least that's how i think it works.
dual cards will split the screen. 1 takes top half and the other renders the bottom.
they will each use thier own memory for the screen overlay resolution.
however thats the easy part. the main card is the only one that is using its memory for allllll them textures and 3d rendering resolutions. so basically yeah, only 1card is pulling the heavy load.
what do y'alls GPUs usually idle at? mine hangs around 133F/56C
under heavy load my hotter card never breaks 40c
i use g10 kraken cooling