Yes, as the one no longer has to drive both 4k monitors. Less memory to show desktop and windows for the one card.
I don’t know about render speed of ‘regular’ viewports. But Raytraced and Rhino Render in v7 will benefit if you set Cycles to use both GPUs as the combined render device. Not quite halving path tracing time, but speed is increased by 1.6-1.8x or so, maybe in some cases closer to 1.9x.