It's impossible.
The exact question was asked on the MSDN forum, and you can read all the answers here .
As Arun said, the TFS 2010 installer checks if the previous version of TFS is installed on the computer and will not allow the installation of TFS 2010 if the previous version is detected.
, TFS 2008 TFS 2010 . , - IIS - "Team Foundation" ".
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--Vladimir