I have a suggestion.
Sharing notes with someone else.

Short explanation:
If 2 users are using standard notes it should be possible to share notes between them.
I can choose to share a note with another user.
That other user can see the note between their own notes. They can hide the note if they want to.

I think it would be best to work with an email.
So if I decide to share it an email will be sent to the other user where he/she can accept the invitation and the note appears in their standard notes. The editing would work like in any other collaboration app. You can see the name of the other person if he/she is typing something.

The owner of the note can choose if the other person can only see or also edit the note depending on the settings.

This function already exists and in Google keep for example and is used in cloud driven office suits.