From what I gather of your post, you're wanting one GPU dedicated to each side of the screen to increase performance of gaming across the entire display?
There are several logistical barriers that make it rather inefficient. For smooth frame times, you want each GPU to be doing roughly the same amount of work. Partitioning the display half and half doesn't mean that the GPU workload is divided evenly in half. Say an explosion happens on one side of the screen and one GPU will have to do all the extra work to render it while the other side continues on. nVidia actually did this before in the DX9 days with an SLI technique called split frame rendering (SFR) but dynamically adjusted the areas each GPU would be wholly responsible for rendering. The dynamic part of how the image was split lead to rather good scaling but it had some compatibility problems with some titles. Alternate frame rendering (AFR) won out as it had greater compatibility and worked with DX10 and DX11.
The other logistical issue is that splitting a displays in half logically requires the drivers to unify them virtually for an application. nVidia Surround or AMD's Eyefinity are necessary to expose the higher resolution of the full screen. Some games do no play nice with Surround/Eyefinity and there is a slight overhead vs. a single display of similar resolution (i.e. one 3840 x 2160 display gets slightly higher frame rates than four 1920 x 1080 monitors in Surround/Eyefinity).
Lastly is the monitors own internal logic for displaying images may not play nice. There is single panel that is being used and it may require each side to update synchronously. Thus if the frame for the left side arrives slightly earlier than the right side, the left side's frame is slightly delayed to render together with the right. This could cause pacing issues and/or image tearing. Worst case is that the display incorporates its own frame buffer to do PBP or PIP and thus everything is delayed on screen by one frame which is not optimal for gaming.
Best practice right now is to simply run the displays at its native resolution and then left SLI/Crossfire do AFR for load balancing. Split frame rendering can make a return with DX12 and Vulkan but that is entirely up to the game developer.
Dual Opteron 6376, 96 GB DDR3, Asus KGPE-D16, GTX 970
Mac Pro Dual Xeon E5645, 48 GB DDR3, GTX 770
Core i7 3930K@4.2 Ghz, 32 GB DDR3, GA-X79-UP5-Wifi
Core i7 2600K@4.4 Ghz, 16 GB DDR3, GTX 970, GA-X68XP-UD4