I noticed a similar thread recently so my apologies if this appears to be a duplicate. The circumstances are a little different so that solution, (a quad graphics card) won't work for me.

I have a client looking to implement 3 monitors on a number of PC's. They want a single desktop spread across all monitors, so the user can determine the layout. I know the quad graphics cardwill do this but the number of machines involved makes that a very expensive option, so I'm wondering if there's a more 'economical' way to do it.

For example, can I run multiple graphics cards in parallel? Can I use a monitor splitter or will that just replicate the image from one screen to the other?