How can the Visual Studio build output location be specified for referenced assemblies?

Question

I have a solution that includes a mix of .NET 3.5 and 4.0 projects. The 3.5 projects cannot be upgraded due to external dependencies.

The solution uses a simple plugin mechanism and I've set the output path on all projects to a common bin folder in the solution folder so that plugin assemblies can be discovered and loaded when debugging.

I have .NET 3.5 projects referencing a 3rd party assembly and .NET 4.0 projects referencing the 4.0 version of the same assembly, which has the same file name as the 3.5 version. When I build, one version of the 3rd party assembly overwrites the other version.

I'd like to have these dependencies output to different subfolders so I can then set the probing private path in config but I can't see how to do this in the build process.