I'm interested in this, too. Both of my properties have 2 entry doors. The entry doors are both on the front of the house, so there's a 50-50 chance a guest will want to enter through the door with the unlinked lock. (Even when I've tried to explain in detail which door their lock code works with!)
Automating only one of the locks defeats the purpose of automation if I have to manually duplicate the code for the second lock.
Either way, it's very confusing and inconvenient to have only one smart lock link with Airbnb.