The problem with eg. INCLUDE=$mac is that you want to change screen resolutions and changing the screen setting needs a reboot. This way you will end up in a contiuous reboot loop.
There is a second parameter called EXIT=ALL which will help you.
Think about this:
You have one wnos.ini with Dualhead=yes and several mac.ini files in the wnosinc folder which have a Dualhead=no configured.
Depending on where in the wnos.ini you have set the INCLUDE=$mac parameter the parsing process will leave the wnos.ini and read the mac.ini before parsing the rest of the wnos.ini.
You have to put the INCLUDE parameter before the Dualhead=yes parameter and put Exit=all at the end of your mac.ini files.
This way the wnos.ini file is read with some default settings, stop processing at the INCLUDE parameter, start reading a mac.ini (if available for that device) and stop the whole process because of Exit=all.
Hope that all makes sense
CG