>>I have both installed separately. You can also upgrade 2008 to 2012 if you'd like. I had troubles installing it and planned to write a blog about it, but never did..
>
>When you had problems installing it, was it a new install or an upgrade? Also, what kind of problems did you have when installing it?

It was a new install. I documented some of the problems here, but this thread in MSDN explains it all
http://social.msdn.microsoft.com/Forums/en-US/sqlsetupandupgrade/thread/2867fe21-6414-4bd9-879e-ded00716ba70