After read this post I though "I'll find the dupe and done" but after search for a dupe about "How to set up dual monitor?" I didn't find any question with acceptable answers.
There are a lot of questions without answers, responses from the OP or any activity since months.
I'd only found this useful two post:
Maybe we can use both post as canonical posts. The first one for ATI and the second for nVidia.
Are both post acceptable?
Are they up-to-date?
Do we need better answers?
Do you know another post that can be used to link dupes?