Exchange 2007 SCR passive node licensing

Hello,

We have 2x windows servers with 2008 R2 Standard licenses and 1x Exchange 2007 Standard license and we want to install Exchange 2007 with SCR.

And the main question is do we need to have 2 Exchange 07 Standard licenses ? 1 for active node (it's obvious) and 2nd for passive node ? Unfortunately I couldn't found any documents on MS pages which concerns this issue.