Buying two (smart) phones and running simcards on both sounds complex and expensive.

Simpler would be a 2nd way to unlock the phone into a fake user account...

I have done this with my laptop for years traveling internationally. If you don't press the right key at the right time you are greeted by windows.

I seem to remember that Android supports multiple user accounts, no?

Sounds like a simple way to end up getting much greater scrutiny in a room somewhere for at least few hours.

This "simple" solution would make my life way more complicated. Not to mention expensive.

I only travel with my backup phone with a fresh re-install.

The main thing is it is nice to get away from the main phone on a trip and enjoy the trip. Losing it would not be the end of the world.

All these other aspects you get for free.

Your second phone could be an old Nokia 1110.

I still have to pay for a second cell phone account, keep it charged, and use up a pocket carrying it everywhere.

have you heard about body searches? where will you keep your two phones?

Doesn’t sound like that is what is happening here. Don’t let perfect get in the way of good enough at least in the short term.

Or install a second OS.

In your mind how does this scenario play out?

Are you just living in the "second OS"? Meaning you don't actually use your phone while you are outside? Or are you going to just casually reboot your phone and boot into the "second OS" while the officer is standing there demanding your phone?

I believe there are some setups where one password unlocks your normal OS and user accounts and another password unlocks your phone under a second user with no indication of a second available.