Sorry to have missed this earlier and the one from last August. This is neither a bug nor a feature, but one of those things that lies between what you thought of and things that don't work.
In other words, this is a use case we haven't considered. We have limited ability to modify Visual Studio projects. Our instructions to people working with Visual Studio is to create your solutions and projects in Visual Studio and then you should be able to add files in SlickEdit. Of course, we still get problem reports for project types that are new to Visual Studio that we still can't work with.
You're better off with two different workspace, here. It seems like these projects must be compiled by different compilers, correct? Otherwise, they'd all be Visual Studio files or non-Visual Studio files. If you are calling from one into the other, then you can tag the one and have the symbols available in the other.
If you have source files from a Visual Studio project that you want to use in a non-Visual Studio project, your best bet is to just add those files to a non-Visual Studio project.
Let me know if this helps.
--Scott