you're looking at having 2 monitors, while using xwindows, and displaying one virtual desktop on one monitor and a different virtual desktop on a different monitor.
I agree that it's an window's manager issue. (Currently most dual monitor setups under X use a single large buffer for display where 1/2 the buffer goes to one monitor and 1/2 the buffer goes to the other monitor.)
However, given how XWindows works (in particular it's DISPLAY setting....it may be due-able.)
Still researching.
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There is an X2X program out there...don't think it's what your looking for though
Mostly, from what I see, you're looking to run dual monitor with xinerama turned off. Then it appears to be just a question of configuring the windows manager to display the appropriate virtual window to the appropriate monitor (using :0.0 or :0.1 iirc).
However, I don't see any windows managers that have been configured for that as yet. :-(
(And I'm planning on getting a multiple-monitor system in the very near future....so this was of vast interest to me.)