This might sound unconventional but I do it with hard edges and blending all the time. Use a DualHead2Go or TripleHead2Go (digital) in 2 output mode.

Don't use the second graphics card output, just the primary.

I am not sure about doing non-standard resolutions like 1400x1050 with the Matrox boxes but when I need to edge blend two HD projectors 1920x1080, this is the solution I use.