How can an ASP.NET Core and React mono repo be opened simultaneously?

This project contains both a ASP.NET Core C# solution and a React application nested in the same folder: https://github.com/sillsdev/TheCombine

If I open the Solution file, Rider automatically detects the C# project and associates the C# files. But if I open the folder as a Project, Rider detects the React project and associates the TypeScript files.

The thing is, it seems like even if I open both projects in separate windows, they seem to conflict. For example, if I open the C# solution after the React project, the React files all stop being highlighted as project files. Is this because there is some kind of conflict/sharing going on in the .idea folder?

I haven't found a way to open both subprojects in this monorepo at the same time with Rider and have the project files be tracked properly.