Horizontal Span is a Windows XP only mode. In vista if you run multiple monitors off of a single card you can clone or have unique desktops (dual view). Multiple cards don't have the clone option as far as I am aware.

I am also using a multiple monitor setup with a very similarly beefy system as listed in the original post. The only real difference is that I'm using a dual core cpu instead of quad core. Running WoW of any size/scale on two monitors instantly reduces the frame rate by 50%. Move both windows back to one screen and the frame rate shoots up. This has nothing to do with your hardware. What I've been able to determine is that this is due to how Vista and D3D work along with the new graphics subsystem. NVidia can't support Horizontal span with Vista apparently due to changes in the graphics subsystems. What may be happening is that all frames are rendered on your primary displays graphics card, and then being output to the secondary screens. The only solution I've found is to run every instance on the main monitor, which is less than ideal.

As for the comments about the 750 being throttled to get people to buy the 790, the reasons to buy the 790 are the difference in components. The things that would effect your base performance given similar cpu and ram are front side bus speed etc. Those are going to be identical. While the 750's may have cheaper parts, I've never seen them work poorly compared to a 790 when using the same copmonents. DDR3 vs DDR2 can have benefits, but for most people they won't notice a difference. More importantly it's not the source of frame rate problem being reported. I have a much beefier system than a friend that is also dual boxing. He gets the exact same reduction in frame rate when using two monitors, we can reproduce them in the same way, and we can remove the effects by moving everything to the main monitor. Honestly, if I didn't have other reasons for using Vista 64, I'd go back to XP 64 for the horizontal span option in the drivers which appears to be the only way to get single card multi monitor 3d acceleration working properly.

Edit: I wanted to note that the frame rate reduction I get has nothing to do with maximizer either. Simply starting to instances of wow in windowed mode manually, and then dragging one to the second monitor results in the drop in frame rate. This points back to Vista/Graphics drivers also.