The followuing could be one method:
Firefox Profile Mamager allows you to select a Profile at Startup.
Set up new FF as Spouse (B) default and Polymath as 2nd profile.
Use Mozbackup in Your Windows acct first to backup your settings and bookmarks (and in case something goes wrong)
In Spouse's windows acct, use Mozbackup to restore the backup to Polymath profile.
So there will now be two independent Polymath profiles, identical at first, in each of the Windows accts.
FF Sync (Tools/Options/Sync) can be set not to merge but always to sync from the FF server. But I'm not sure if it handles the profiles correctly or is 'global'.
Have a play, if you used Mozbackup, it's easy to recover if you mess up.
Otherwise just Sync by a manual method/app every now and again.
nc (Tools/options/Sync can be set