I would start by making sure that API.cs and program.cs are part of different projects. The way I open projects is I pin Visual Studio to the taskbar (you should pin both of your versions). Once you've opened the correct project right right-click on that version of Visual Studio in the taskbar and pin the project to the recent files list. Then all you have to do going forward is right-click on the appropriate version of Visual Studio and left-click on the desired project.

If you want to make your two versions of Visual Studio look vastly different you can customized their icons: