From what I read you need to have a paid account in order to be able to have multiple accounts in the protomail app.
Also fairemail or alterantives don't work.
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Download the ProtonMail apk from a trusted place such as APKMirror, and then use Revanced Manager to change the package name. Install the patched app and profit! You can do this over and over again.
I guess a web app manager like native a would work but I suggest paying for an account.