Developer reporting in

vid_netwmfullscreen 1 in the console (shift + esc) solves your problem with streching the fullscreen game to two monitors (vid_restart is needed after you set the cvar).
Unfortunately, this is undocumented, I've already mark this on IRC to fellow devs.
Also, to my experiences this makes the game to loose a few FPS, but not too much, 10-5 in avg, and, it *might* crash on wm events, but blame this on the bad NVidia drivers, not on the game (I've had several segfaults in the past, however I keep my drivers updated, and haven't seen a crash in a while).

8400GS should be enough to run the game at least on normal video settings, +blurry reflcetions enabled.