The lspci command will provide you with most of the info you need to put into the X configuration file.
at a shell/terminal command prompt.
You will probably need to be root to do that.
If both cards are not showing there on the list run
which will probe for new hardware and attempt to include the new card. The kudzu
is much like what winblows does with the "Add New Hardware" widget but quieter.
(PS: The hardware browser sux.
Don't use it for anything except very general information.)
Assigning hardware address only changes which monitor gets the display. The only tricky part is assigning the "Screen" & "Device" info correctly in the "ServerLayout" configuration, but it is not
hard - might take some trial & error tests to get it perfect.
Before changing the Xorg.conf file be sure to make a backup.
Before making any changes, make a little drawing with two boxes on it. Under, in and around each box design
how you want the setup to be. Then you can be relatively certain that what you want is what you get.
Happy New Years!