TwinView будет работать для ATI?

This method of multiple monitor use has several distinct advantages over other techniques (such as Xinerama):

* A single X screen is used. The NVIDIA driver conceals all information about multiple display devices from the X server; as far as X is concerned, there is only one screen.

* Both display devices share one frame buffer. Thus, all the the functionality present on a single display (e.g. accelerated OpenGL) is available with TwinView.

* No additional overhead is needed to emulate having a single desktop.

Сравнение производительности (выигрыша от one frame buffer?) twinview vs xinerama:

http://www.phoronix.com/scan.php?page=article&item=387&num=5
http://www.phoronix.com/scan.php?page=article&item=387&num=6
http://www.phoronix.com/scan.php?page=article&item=387&num=7

Похоже, что для Radeon это называется MergedFB:
http://dri.freedesktop.org/wiki/MergedFB

The current Radeon dualhead code creates two separate screens with separate framebuffers. They can be used separately as separate X servers (host:0.0 and host:0.1).

Так все-таки
(http://www.linux.org.ru/view-message.jsp?msgid=3830927)
можно ли запустить несколько X-серверов одновременно?

А вперемешку с консолями?

http://www.botchco.com/alex/radeon/mergedfb/cvs/DRI/final/radeon.4.html

Option «NoMergedXinerama» «boolean»
The default value is FALSE.

Merged framebuffer mode provides its own pseudo-Xinerama. This allows Xinerama compliant applications to place windows appropriately. There are some caveats. Since merged framebuffer mode is able to change relative screen sizes and orientations on the fly, as well has having overlapping viewports, pseudo-Xinerama, might not always provide the right hints. Also many Xinerama compliant applications only query Xinerama once at startup; if the information changes, they may not be aware of the change. If you are already using Xinerama (e.g., a single head card and a dualhead card providing three heads), pseudo-Xinerama will be disabled.
This option allows you turn off the driver provided pseudo-Xinerama extension.

TwinView - опция, работающая только в проприетарных nvidia-дровах.