Nope, the 4670 will be sitting there doing nothing but consuming a bit of power, the second monitor is a minimal load on the 6790 so even offloading it to the 4670 wouldnt gain you anything but more heat.
Actually, there is a small benefit if you run a 2nd monitor off the other card. The primary card would no longer run at full memory clocks while idle. For a single card to run two monitors requires the memory to operate at full speed, and usually raises idle temps by about 10C or more. With 2 cards, they both can completely run at idle clocks. Of course the benefit is a rather dubious one, and I wouldn't advise doing it, unless you were also using it as a physX card.