2 PCs, 4 monitors: syncing them WITH window drag-and-drop functionality?

I have 2 computers in my office (one with Windows 7 and the other with XP Pro). Each of these computers has 2 monitors. In total, I wanted to use these 4 monitors as if they were connected to 1 single computer, so I downloaded Input Director but all it did was sync the monitors. For the most part, this gets me by…

Here’s the problem: When I try to drag an open window or an icon from one set of monitors belonging to one computer over to another set of monitors belonging to the other synced computer, well, Input Director just can’t do it and stops you at the edge of the farthest monitor’s edge. Also, unless you have a shared folder being utilized between both computers, Input Director can’t copy-and-past stuff between computers (unless we’re talking about generic text you highlight and copy from some website or something; that, it can do).

Is there any program out there that can basically connect 2 or more sets of displays, sync them, but also make them synthesize the experience one has when dragging one window over to another monitor when both monitors are connected to 1 computer?

Connect all four monitor to one computer…done.
What you want cannot be done with software.

So you’ve used Multiplicity Pro from Stardock?