One of the many display modes offered by the 
NVIDIA range of graphics drivers. Normally associated with multi monitor configurations or TV output on the GeForce seris of cards.
More information can be obtained by looking at the PDF manuals avaliable for download at the NVIDIA WebSite, Linux3D wiki TwinView page, and our NvidiaTwinView and NvidiaDriverHowto pages.
3 pages link to TwinView:
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