You are confusing things, the idea of having multiple main() is fundamentally wrong. Libraries never ever contain a main() function.
(With the exception of Windows DLLs that contain a DllMain, but that means something different.)
If you want to provide a test case for your library, you make the test case as a separate project which includes the library. The test code should not be inside the library itself, neither should main().
Also, I very much doubt you are able to build a program with several function definitions that have the same name, be it main() or something else. If you believe you have managed this, I would either suspect that you haven't linked the files correctly, or that the linker is crap.