You have to bind the files 2x. Once on WIndows, once on Mac using the same binding key.
Well, I think the runtimes could simply share the database.
You have to have FileMaker Advanced for Windows and for MAC. I am working with this right now. Follow the instructions in the Advanced guide. Bind the files first on Windows and then bind the files on the MAC.. I will tell you that you need to check the "Overwrite matching files within the Project Folder" box. This was my biggest han up when learning how to do this.