As far as I know the only way you could do this is if both users purchased and installed a copy of the app each, then User A backs up their collection to the cloud and gives the cloud login to User B. User B can then use the login to 'restore' the backup to their own device. Pretty straightforward, but it relies on you trusting User B to not overwrite the backup.
Maybe @Support can confirm if there's other way to remotely share a collection?
