Someone on Apple’s support discussion group claimed “this can’t be done.” Apple explicitly supports shared calendars, but not shared contacts. They probably haven’t figured out how to deal with two people updating things at the same time.
Then I found this terrific article by Lena Shore. Most of us have our contacts saved in a “personal” iCloud account. Shore’s approach is to set up a “household” iCloud account. You save the shared contact list to the household account. Then you enable Contacts under both accounts. Everyone who does this will see both their personal list and the household list.