As has been discussed many times, there is not a copy of a DEVONthink database in any sync location.
Also, you should not and cannot put a DEVONthink database in any cloud-synced folder or you could irreparably damage it.
We have a shared iCloud account
Meaning you’re both logging in with the same Apple ID?
Are you often changing the database and needing to share frequent updates? If not, then why use a remote sync option?
You could use a Bonjour sync on your local network and the devices would sync when they’re both on and running DEVONthink.
There is a forum post on syncing: Sync Types Explained. This is a good place to start.
There is also one specifically about Bonjour: Bonjour Simplified