Problem importing an IDL file into another IDL file

I have an ATL project, containing two simple objects.

I manually defined a couple of extra interfaces in IDL, and they work if I put them in the original IDL file. However, if I put them in a separate IDL file and import them into the original, they compile OK but no .H file is generated for them, and when the .H file generated for the original IDL file attempts to include their .H, the build naturally fails.

Can anyone suggest what the problem is?

Thanks, RD

Visual Studio assumes a project will contain just one IDL file, and defaults the output file names accordingly. This behavior can be customized, under the file's properties in Solution Explorer (MIDL properties): change the header, IID, proxy and type library filenames to the appropriate values. Afterward, be sure to include the generated dot-H file where needed to make use of the compiled definitions.