EDIT - Read this that they are getting a headset each, if not, then most of the below doesn't apply. Mostly the 1st and final paragraphs. Though app sharing stuff still applies.
Best way would be for the first one to use someone else's referral, if you have a friend with one etc, then the second one uses the first referral. If you don't have a friend with a headset, then obviously ask here and there's plenty of people happy to get £23 credit too
You can also then use the primary account to log into both headsets, and then they can share apps. That way only the primary account holder needs to purchase the game, then the secondary can still use those games to play.
You may need to factory reset the 2nd headset after setting up the referral on it, so you can then use the same account on both as the primary.
I have 2 headsets (2nd bought 2nd hand, so no referral), and have myself as the primary on each, then my wife as the 2nd. This allows my son and I to play games together, without making any purchases on my wife's account.
I would suggest letting whoever is going to use it the most to have the £46 credit, buy all the basics on that one, and share with the other, then if there's other experiences the 2nd account wants, they can use their credit to get it.
One more thing to consider. A 13 year old will not be able to purchase quite a few of the games/experiences without parental consent, but someone using your wifes account will have access to everything. Might want to be careful that the kids aren't getting their hands on things they shouldn't.