Not only this - my observation is that having a secret backup phone is not an unusual practice for kids who might get their primary phone taken away at times by parents, school officials, etc. Or if their primary phone is subject to technical parental controls.
Many of your high school aged friends had a secondary backup phone with a separate cellphone plan they pay for? That’s wild!
I don’t think the average American high school student has two smart phones one of which is a secret from their major source of income (their parents).
No, why would they pay a cellphone plan for it? For most of them it was their or their siblings' previous phone, or their first phone they paid for themselves when they got a job, for others it's a cheap 60$ used phone they bought when their parents took theirs away at some point. That's like money for going out for lunch three times.
There's no use for a cellphone plan, we would just use wifi or hotspot from their main phone.
Yes, anecdotally many of my friends (and I) had a backup phone. Second hand Android phones are so cheap that why not? By the time the app has to be reinstalled it may make sense, if sideloading on Android takes off.