I think there are two different ideas here (but I may not be understanding):
- changing the config file so both tablets look like the same tablet to the cloud
- copying a notebook from one tablet to another one.
Regarding 1.
I did this: Two Tablets One Account, and I'm glad I did. But if you still have stuff scattered over two accounts, you'll want to make backups first. There are instructions in various places for ssh (command line) access, (e.g., reMarkable Guide, Information about reMarkable 2), but if that all still looks Greek, I hear you. Which leads to ...
Regarding 2.
RCU is a fabulous utility program. You buy it ($12), you install it, and you configure both your tablets in it. You can make backups by connecting RCU to the tablet, then Notebooks => Select all => Download, choose a folder and it saves your notebooks in a format that can be copied back to the tablet or to another tablet.
On the other tablet, you connect to RCU and Upload a saved file from the other tablet's backup folder. In my experience, there isn't another utility that allows you to do this as easy as RCU. It does a whole bunch else besides (e.g., custom templates and wallpaper), but backups and copying files have been my main uses of it--though I do use its virtual printer, i.e., I can print to the tablet.
In summary, you can do 1. if you really want both tablets to always contain the same stuff. Or you can do 2. for easy copying from one tablet the other. Either way, you should do 2. for easy backups.