0triple monitor / dual video card set up

[Seb <spluque@gmail.com>]

Hello,

I've recently moved onto a setup with:

* 3 monitors

  Resolutions:

  - 1920 x 1080 (2)
  - 1680 x 1050 (1)


* 2 videos cards

  - GeForce GT 720
  - RV730 Pro (Radeon HD 4650)

The two monitors with the same resolution are plugged into the GT 720
and the remaining is plugged into the HD 4650.

After a lot of effort I was able to get the monitor(s) from either card
to function properly with the attached /etc/X11/xorg.conf.  The
"Default" layout works well whether I use the Screen section from either
video card.  Unfortunately, it fails whenever I attempt to combine the
screens.  In the latter case, only the Screen section that is listed
first gets displayed properly, and the other shows a black screen(s)
with an X-shaped cursor whenever the cursor wonders into the area.

Any tips on what is going on?  Have others been successful in setting up
a config file for a multiple video card system requiring a mixture of
drivers like nouveau and radeon?

Thanks,

-- 
Seb

Section "ServerFlags"
    DefaultServerLayout    "Default"
EndSection

# Left monitor
Section "Monitor"
    Identifier	"Dell"
    # HorizSync	30.0 - 83.0
    # VertRefresh	56.0 - 75.0
EndSection
# Center monitor
Section "Monitor"
    Identifier	"Samsung"
    # HorizSync	30 - 81
    # VertRefresh	56 - 75
EndSection
# Right monitor
Section "Monitor"
    Identifier	"Acer"
    Option	"RightOf" "Samsung"
    # HorizSync	54.2 - 83.8
    # VertRefresh	49 - 75
EndSection

Section "Screen"
    Identifier	"Left"
    Device	"AMD/ATI (Top Slot)"
    Monitor	"Dell"
EndSection
Section "Screen"
    Identifier	"CenterRight"
    Device	"NVIDIA (Bottom Slot)"
    Monitor	"Samsung"
    SubSection "Display"
        Virtual    3840 1080
    EndSubSection
EndSection

# Secondary card
Section "Device"
    Identifier	"NVIDIA (Bottom Slot)"
    Driver	"nouveau"
    VendorName	"NVIDIA Corporation"
    BoardName	"GeForce GT 720"
    BusID	"PCI:3:0:0"
    Option	"Monitor-DP-1" "Samsung"
    Option	"Monitor-DP-2" "Acer"
    # Option	"ZaphodHeads" "DP-1"
EndSection
# Primary card
Section "Device"
    Identifier	"AMD/ATI (Top Slot)"
    Driver	"radeon"
    VendorName	"AMD/ATI"
    BoardName	"RV730 Pro (Radeon HD 4650)"
    BusID	"PCI:1:0:0"
    Option	"Monitor-DVI-0" "Dell"
    # Option	"ZaphodHeads" "DVI-I-1"
EndSection

# Server Layout
Section "ServerLayout"
    Identifier	"Default"
    Screen	"CenterRight"
EndSection

# Not working: Left is showing only X cursor and some garbage at edges
Section "ServerLayout"
    Identifier	"Full"
    Screen	"CenterRight"
    Screen	"Left" LeftOf "CenterRight"
EndSection