[modeless]

I doubt you could make the border smaller than a pixel; pixels are tiny these days. Having worked with display walls in the past, even a 1-pixel border between displays is really distracting, to the point where I'd rather just use the 2 displays separately as in a normal dual-head setup.

If you really need a borderless display the way to do it is with a projector. You can seamlessly combine projectors to create a display as big as you want. Normally the projected images are overlapped a few pixels on each side and blended together to cover up alignment issues. I'm certain this is how they make that enormous display that towers behind Steve Jobs during his keynotes at Moscone Center.

If you're interested in joining the displays from multiple computers together, check out http://code.google.com/p/synergy-plus/ . It's like VNC in reverse.

Also, X Windows theoretically allows moving running application windows from one device to another but I don't think any implementations really support it.