The 2 issues I'm seeing (from DP1 to now) are both Safari related.
1. Sometimes a safari tab will have loads of wierd artifacts (like when a graphics card crashes). Reloading the page makes it go away
2. When using an 'in browser' editor such as google docs, the further the curser goes to the right, it starts being inaccurate, so you type but the text comes out 3 or 4 charachters to the left of the cursor - very strange!
Not sure if they are related to being on dual screens. Other than that I've not had any real issues.