How to get dual graphic cards working under Linux?

As far as I can tell from your problem description the correct term for what you want is "hybrid graphics" as you only use either adapter to power your monitor (actually any output), not both at the same time.

An overview over tools for what you want can be found (for example) here. You may be looking for the tool bbswitch from the Bumblebee-Project. Yet, you should not put anything about drivers into your xorg.conf. X should find and load the correct drivers itself. Also, I remember reading something about improved support for hybrid graphics in the most recent stable kernel, so you should get/build the most recent stable kernel for your distro. You may find more on the internet by the term "hybrid graphics" anyways.

Hope that helps.