How to merge WhatsApp backup from old phone to same WhatsApp

I have recently migrated from a OnePlus3 to a OnePlus8Pro. The problem is as I have been toggling between the two phones and utilising What'sApp on both (having to wait to verify device), What's App on my new phone had an older back up, and whilst I've been trying to resolve this by going between the two phones, I have been acquiring new messages on both devices when I am on a particular device. I thought that backing up my messages on my old device, then going to my new one would mean that I could merge all the messages by restoring the backup I had just done on my old phone, but I have no idea how to do this or whether this is indeed possible. What a palaver. I feel that this should be simple but What'sApp doesn't appear to have made it so. Any help would be gratefully appreciated as I'm the kind of person that likes to keep his messages and ideally of course all in one place.

Windows / Firefox 109.0