SQL Server 2012 + SQL Server 2008R2 on the same VM
We have mixed instances of SQL 2008R2 and 2012 on the same server running either windows server 2008R2 or 2012.

there are no problems at all and it is supported by Microsoft.

note that we are a company that make software for investment banks and our software is used in biggest and critical deals like Facebook, twitter, etc globally and we do hosting for different investment banks that use our software.