https://github.com/dotnet/cli/tree/master/src/Microsoft.DotN...
https://github.com/dotnet/cli/tree/master/src/Microsoft.DotN...
c# is used to bootstrap all tools, so it's the first supported language and the default. Nightly, dev build have f# support, but it's a beta ( merged last week, but it's ok ).
f# dotnet cli (the wrapper for f# compiler) is a beta, but moving fast, not much todo. For example the f# support in dotnet new is in a pull request.
The F# compiler built for coreclr ( bundled with dotnet cli, so ready to go ) is working pretty much, some stuff missing ( portable pdb for example ),
f# coreclr is open source https://github.com/Microsoft/visualfsharp
You can have multiple project (same repo, different directories ) and reference one from another
you can have each project.json use a different compiler setting the compilerName property ( csc for c#, or fsc for f# ) .
A project can reference another project, so you can reference a F# project in a c# project, or the other way around.
like now in .net with msbuild or other tools