You should keep them both as part of application.css and do the following
application.html.haml/erb
body{:class => @old_layout ? "old_layout" : "new_layout"}
Then when ever you are in an action that is off the old layout in your controller put
@old_layout = true
Then in your css files either prepend everything with body.old_layout or body.new_layout or use scss, a subset of sass, and rap your current css files like so
body.old_layout{
#all your old layout css goes here, all but body statements of course
}
body.new_layout{
#all your new layout css goes here, all but body statements of course
}
This way you keep things simple with one css file. And with a solution that easily allows you to switch over each controller action one at a time.