I was going to suggest localsyncstore, but turns out that won’t sync with the mobile devices… have a look here to see various sync options: Setting up syncing - best way?
If you don’t want to use a cloud-option, then I think Bonjour would be the only way. I have never tried syncing a device with itself (i.e. two accounts on the same device); certainly both accounts would have to be logged in, both DT apps running; if that is actually possible, then setting up Bonjour is a doddle - on one account go to Preferences/Sync, click Bonjour options, Enable incoming connections, provide a password. Now on the other account (leaving the first account open and DT running) go to Preferences/Sync; a local network sync should appear in the locations list; select it, and you should be able to see the databases which are to be synced. You will be asked to provide the password you used earlier on. You would want to select the database the two of you share.
There is no database password, so there would be nothing stopping you from selecting you wife’s databases and syncing them to your account and vice-versa.
In a second step, I guess you will need to set the second account to accept incoming connections too - because you want to sync the mobile device with the account. On the mobile device you should then see two local network locations; for one mobile device in the one location you will want to select the database you both share, in the other you will want to select the databases for account two. In the second mobile device you will only want to sync from the first account you set up.
Does that make sense? It’s what I would try, anyway. Perhaps it will even work 
Otherwise perhaps other kind souls will provide more input here.
PS I would choose to have a backup of my databases prior to setting out on this mission; I’ve never had DT corrupt anything, but prior life experience suggests multi-way-sync could be a good way to destroy a database with a lesser piece of software.