Using 2 GPU's can be as expensive as a hardware related solution and for high demanding tasks such as working with more than 2 monitors it would be a better idea considering a solution like the Matrox TripleHead2Go
Which seems to be plug-and-play and the windows/mac compatible software is mentioned for special arrangement/management of your video setup.
Product Features and Technical Details
- Run two or three monitors from your laptop or desktop computer
- 1 VGA input, 3 VGA outputs
- Works with LCDs, CRTs and projectors
- Supports a maximum resolution of 5760x1080 (3x 1920x1080) across
three monitors, or 3840x1200 (2x 1920x1200) across two monitors;
displays must run at the same resolution
- Includes one 2-foot HD-15 (analog) monitor cable, one 2-foot DVI-I
to VGA cable, external 5VDC power adapter, region-specific cable(s),
and software CD
- Brand Name: Matrox
- Model: T2G-A3A-AJF-P
- Item Package Quantity: 1
- Graphics Coprocessor: Matrox
I am not sure (and you don't mention) what you are going to do with your monitor arrangement, which leads to useful things like audio/video production, programming, and video gaming.
However, using more than 1 GPU may not result in what you expect on performance, different than using only 1 GPU and an external hardware like this. And a solution can be as expensive as the other one. (from $299.99 on BH Photo & Video, more expensive in other places)
I haven't tested this hardware myself which is indeed cheaper than other hardware. I am using another good solution in order to Perform Multi Monitor real time video-wall using digital inputs but I am talking of a Broadcast Video Mixer similar to this Sony Anycast Station AWS-G500
I am following your question so close in order to find better (and of course: cheaper) solutions, but what I can share is related to broadcast systems, which is high demanding and -sorry- expensive.
I hope you find a solution soon and if so, please share it with us.