I would say that if this plan is not possible, then how about a way for the apps to have a way to sign each other in, especially for those that have multiple services setup. For example:
Phone/tablet could have Mail, VPN, Pass. If a user signs into one, the other apps when next opened without a signed in account, will then prompt asking if you would like to use the account login from app xyz
Same could apply on desktop too
I would say that if this plan is not possible, then how about a way for the apps to have a way to sign each other in, especially for those that have multiple services setup. For example:
Phone/tablet could have Mail, VPN, Pass. If a user signs into one, the other apps when next opened without a signed in account, will then prompt asking if you would like to use the account login from app xyz
Same could apply on desktop too