i do not think thats possible with twinview, but i think it is possible with xinerama.
first, read 'Chapter 13. Configuring TwinView'. i do not see anything related to multiple gpus. the "only" reason to use twinview over normal xinerama is that with twinview all displays can show hw-accelerated opengl. so, imho, xinerama is the way to go.
read 'Chapter 15. Configuring Multiple X Screens on One Card' to find out about how to setup multiple 'screens' (a 'screen' is connected to a 'device'[gpu] and a monitor, so basically a 'screen' equals one of your monitors). without any further tuning you will now have one xserver running which shows several 'separated' areas (you can't move windows between them).
Section "Screen"
Identifier "left_screen"
Device "left_gpu"
Monitor "left_monitor"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1600x1200"
EndSubsection
EndSection
Section "Screen"
Identifier "middle_screen"
Device "left_gpu"
Monitor "middle_monitor"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1600x1200"
EndSubsection
EndSection
Section "Screen"
Identifier "right_screen"
Device "right_gpu"
Monitor "right_monitor"
DefaultDepth 24
Subsection "Display"
Depth 24
Modes "1600x1200"
EndSubsection
EndSection
you add up all 'screens' until you are satisfied with your layout (you can have multiple layouts, distinguish them via the Identifier statement):
Section "ServerLayout"
...
Screen 0 "left_screen"
Screen 1 "middle_screen" rightOf "left_screen"
Screen 2 "right_screen" rightOf "middle_screen"
...
EndSection
now read 'Chapter 14. Configuring GLX in Xinerama'. essentially it says: put
Option "Xinerama" "True"
to your Serverflags section.