ttp://www.microsoftstore.com/store/msusa/en_US/pdp/Visual-Studio-Professional-2015/productID.323825200
> The software included in the license is the current version of the software, Visual Studio Professional 2015,
> plus downgrade rights to simultaneously run prior versions of Visual Studio Professional　to which you may otherwise have access.

rights to simultaneously run prior versionsだから、VS2015と古いVSを同時に使えるって意味じゃないな