What's the reccomended way to keep resource ID's synchronised?

The only way to do it is to have the value in only one header that is included in both places. I do this and it is working well for me when I do this.

One thing to consider is do the DLL's resource IDs really have to be visible in the app's resource file? I can't think of a case where they do. I realize it can make some things much more convenient but it is not absolutely necessary. This means you should try removing the inclusion of the library's Resource.h file from the app's resource file. Resource IDs can be the same in the DLL and the app and there are some functions that allow you to select the handle to use for resource lookups which resolves those issues.

In my newer apps I make a special effort to have the resource IDs visible to as few modules as possible. In fact, my dialog handling code does not have any header files. There is an interface defined that does not involve ANY resource IDs. If you think about it, why should there be? Only the specific code that manages the dialog needs them. The code that invokes the dialog doesn't. All it needs to do is call a function and pass it some data and the parent window. The dialog handling code manages all of the data for the app. The app doesn't need to know about every control in the dialog and what type of objects they are or anything else about the dialog like its ID. That doesn't matter to the app because it doesn't use the ID for anything.