Merge Tables is a part of my Sync Tables Pro pack, just saying 
It was announced and implemented in the first demo and didn’t yet make it for the soft launch. It’s 99% ready — I just wanted to examine it for the ways to make the UX better. So it’s coming sometime this week.
The plan for the feature is to merge any number of tables, allow any number of merge tables in a doc, and also have all the benefits of Sync Tables Pro pack such as row-level security and swappable sources.
P.S. I’m sorry I missed your thread. I would’ve notified you that I was working on it too.