I was wondering what method you @Stephen_C now use to simply link two PDFs in the DT3 database.
Like you, I am looking for a simple way to link two (or more) PDFs and have the links appear in the Generic Inspector. It is feature that I used in Zotero.
If there is no simple method, we should put a request for a feature in future development of DT3.
Cheers! 
There are very few files that I need to link so I’ve used the annotations method. It’s not ideal (for me) but it’s the best, simple way that I took from the suggestions given in this thread.
Stephen
Vote + for this feature as I already requested here too Linking documents for contextual reasons
This would really help a lot for cross referencing. There is so much factual possible in DT but sometimes it lacks of implementation so you can use it in an standartized intuitive way without custom efforts and side effects.