NVPlatformProbe routine does not mark the entity as Shared for ZaphodHeads.
It is used by xorg since version 1.13 so there is no possibility to use
ZaphodHeads.

This config:
 75 Section "Device"
 76     Identifier  "CardA1"
 77     Screen    0 
 78         Option     "ZaphodHeads"  "DVI-I-1"
 79     Option  "Monitor-DVI-I-1" "Monitor0"
 80         Option     "NoAccel"    "off"           # [<bool>]                  
 81     Driver      "nouveau"
 82     BusID       "PCI:1:0:0"
 83 EndSection
 84 
 85 Section "Device"
 86     Identifier  "CardA2"
 87         Option     "ZaphodHeads"  "VGA-1"
 88     Screen     1
 89     Option  "Monitor-VGA-1" "Monitor1"
 90         Option     "NoAccel"    "off"           # [<bool>]
 91     Driver      "nouveau"
 92     BusID       "PCI:1:0:0"
 93 EndSection
causes xorg to print "Requested Entity already in use!" and use PCI:1:0:0 as
single-head adapter.

Thanks.