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I have the beta app on my second iPhone but don't see how to trigger the QR code screen in the setup flow. It only asks for a phone number to proceed.

Whereas on Android, it has an additional option during setup to "connect to linked device" which allows setting it up in companion mode.

How to trigger companion mode on Whatsapp on iPhone?

It just showed up for me randomly one day. I think I deleted the App again on my 2nd iPhone and after installing it again, the option was there
 
It's really annoying because I'm on the Mac beta but it's a different build for iPadOS.
 
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