you can force the core being used by a program pretty easily. Try StartAffinity.
Command Line Examples:
startaffinity notepad 0
(would run notepad on cpu 0)
startaffinity notepad 1
(would run notepad on cpu1)
startaffinity notepad 0 1
(would run notepad on cpus 1 and 0.)
Just drop the exe in your windows directory then modify your shortcuts to include startaffinity for anything you want to set the mask for.
Hmm.. i just realized this may be a little tough if you have the steam version - unless you can launch it outside of steam itself..