nvidia - Can't get second GPU recognized

I just stuck in another GPU to run two screens.

$ lspci | grep VGA
02:00.0 VGA compatible controller: NVIDIA Corporation G86 [GeForce 8500 GT] (rev a1)

03:00.0 VGA compatible controller: NVIDIA Corporation GT218 [GeForce 210] (rev a2)

but running xrandr I only see the first one:

$ xrandr
Screen 0: minimum 8 x 8, current 1920 x 1200, maximum 8192 x 8192
DVI-I-0 disconnected (normal left inverted right x axis y axis)
VGA-0 disconnected (normal left inverted right x axis y axis)
TV-0 disconnected (normal left inverted right x axis y axis)

DVI-I-1 connected 1920x1200+0+0 (normal left inverted right x axis y axis) 518mm x 324mm
   1920x1200      60.0*+
   1680x1050      60.0  
   1600x1200      60.0  
   1440x900       75.0     59.9  
   1280x1024      75.0     60.0  
   1280x800       74.9     59.8  
   1024x768       75.0     60.0  
   800x600        75.0     60.3  
   640x480        75.0     59.9  

And running gksudo nvidia-settings I only see the one GPU.

I found something about enabling two GPUs in the nvidia drivers so I tried this:

$ sudo nvidia-xconfig -a

Using X configuration file: "/etc/X11/xorg.conf".
NVIDIA: could not open the device file /dev/nvidia1 (Input/output error).


WARNING: Unable to use the nvidia-cfg library to query NVIDIA hardware.


ERROR: Unable to determine number of GPUs in system; cannot honor '--enable-all-gpus' option.

Backed up file '/etc/X11/xorg.conf' as '/etc/X11/xorg.conf.backup'
New X configuration file written to '/etc/X11/xorg.conf'

Any help highly appreciated!