The parameter I'm thinking of specifically would be one username with multiple passwords such that it's not obvious to another person that there are multiple accounts.

I've been thinking a lot about US border agents and law enforcement generally demanding to have access to people's digital devices and though I think it would be great for everyone to take a stand and refuse, it seems like this could mess up your day pretty bad. If you could give a different password that would then show them a generic or empty account so you can be on your way it would probably go a lot smoother. You could also have this dummy account signed in to generic social media accounts if you wanted to go this step.

How possible is it to build something like this on different platforms (win/mac, android/ios)?