I'm not so sure you need to install the two versions. You could put 2012 on and set the compatibility level for the 2008 databases accordingly. The manual states its not a total backwards compatibility with prior versions but running two entire servers on one box is going to cause issues even if it is only service patch conflict and contention for disk access.
http://msdn.microsoft.com/en-us/library/bb510680.aspx - what Microsoft advises