The only way to install both versions of V-Ray is to install them for different 3ds Max versions. For example, V-Ray 2.0 could be installed for use with 3ds Max 2014 and V-Ray 3.0 could be installed for use with 3ds Max 2015.
Another option requires both 3ds Max and 3ds Max Design to be installed (same versions, for example 2015). Then V-Ray 2.0 could be installed for use with 3ds Max 2015 and V-Ray 3.0 for 3ds Max Design 2015. This is possible because technically they are two different applications, regardless of the fact that their functionality is practically the same.
Please note that there is no way to install both V-Ray 2.0 and V-Ray 3.0 for use with a single 3D application. This is because V-Ray modifies and uses the 3D application's sources, the application paths, and the configuration files.