Here is the feedback from Bosch via official helpdesk ticket:
Thank you for contacting Bosch eBike Customer Service New Zealand
I am really sorry, but the Lock feature can only be paired with one phone at a time. I understand how inconvenient that this must be for you, but at the moment it is only one phone.
I'll ask our IT department (in Germany) if we have anything coming or are working on anything for the future and will reply to this email chain once I have an answer. - hopefully in the next 24 to 48 hours.
Perhaps unsurprising really.
I think this actually creates a security risk - given multiple family members would like to ride this bike I suspect the engine/app lock will be turned OFF now, and highly likely my wife does not remember to manually turn it back on when she is out and about, relying only on physical security lock measures.
In a perfect world, the bike engine should be disabled anytime it is powered off, and can be unlocked by any smartphone of authorised users (e.g. family members, trusted friend etc).
Now, I'd imagine, we lose the benefits of engine lock as it's too restrictive/limiting to only one user.
Cheers
Sam