[jen20]

> Basically a single assembly has to be written in the same language.

This is actually untrue - though the tooling does not make it easy.

If the output type of a 'project' is set to `module` instead of `assembly`, it can be added as a module reference to another project which builds an assembly.

The downside used to be that completion in the IDE didn't work, but as far as I'm aware there's no real reason why multi-module assemblies couldn't easily be exposed by tooling if it was considered important enough.