There’s some confusion here, iMessage is end to end encrypted by default. That in no way protects the information on each users device.
If iCloud is enabled, then by default it gets unencrypted copies of these messages from the device unless “advanced data protection” is also enabled which ensures iMessage is encrypted but means losing your password also loses access to these backups. However, disabling iCloud sidesteps this issue and honestly if you want that kind of privacy then disabling iCloud is probably a good idea.
So if one users uses ADP and the other user disables iCloud then the conversion is protected.
If you scrolled down in the link provided above, it mentions with BOTH Standard and Advanced data protection messages are end to end encrypted, it’s just with advanced data protection the encryption key also ends up being encrypted too, but I’m positive even this has changed recently. You can try looking at logs when you turn on messages in iCloud and see that your messages are encrypted.
So lots of confusion in this thread, my advice for Apple would be make it very very clear to users that your data is safe. I mean they are threatening to back out of UK, so it’s against their core principles and also probably very technically expensive to undo they end to end encrypted system.
If iCloud is enabled, then by default it gets unencrypted copies of these messages from the device unless “advanced data protection” is also enabled which ensures iMessage is encrypted but means losing your password also loses access to these backups. However, disabling iCloud sidesteps this issue and honestly if you want that kind of privacy then disabling iCloud is probably a good idea.
So if one users uses ADP and the other user disables iCloud then the conversion is protected.