I just test dual display with separate X screens - gears from Mesa/glut and nv_VAR demo -
it works fine...
glxinfo return full information from 2 screens...
Athlon TBird 750@1035,
2xPhilips 170b2(17"TFT - #1 with VGA cable, #2 with DVI-D cable)..
RH8.0 default(2.4.18-18, X 4.2.0, etc)
but I had a problem, when second monitor was connected with DVI-A(analog) - it doesn't start at all (try to set 1024 instead 1280)
Hopefully, this info will be useful for you...