I’m not quite sure I understand how you are trying to tie in module definitions with the module instances.

Perhaps this is what you are trying to do:

Place a module instance on a page in your site which we’ll call first page.

Now go to a second page in your site where you want to add the (same) module. On this second page in the Admin CP select add an existing module, in the DD select the first page where the module is, then select the module and then add the module instance to the second page.

You now have the same module on the second page.

The module can now be edited on either page. With the exception of the module title, the settings and content will propagate for both pages.

Hope this helps.

Regards
Mike