Multi-User Address Book Sync

I have 2 user accounts on the same mac, and I want them to share the same address book via iCloud. Each Mac user account is logged into the same iCloud account. Each user's Address Book is linked (in prefs-accounts) to the same iCould account. Each user has the iCloud source selected (on the Groups page) and each user sees the same Address Book info on the iCloud web site. But... when each user opens the Address Book app from within their respective user account, the data is not synced, and changes made by one user are not synced to the other user - even though I THOUGHT they were accessing the same iCloud file. I am confused and stumped.