They would both have to leave whatever family sharing groups in which they were a part. Then one could set up Family Sharing with the other. You can only be part of one family at a time and can only change once per year.
Note that when changing email addresses, you do NOT need or want to set up a new account. You change the email address on the old one. Unfortunately now that you have set up a new account and presumably with the new address, it will be a real mess using that address for the old account. It is possible but I would have to spend a lot of time explaining how so I'll wait to see if you really want to know.
The support article Change your Apple Account primary email address - Apple Support outlines various methods for changing the email address (Apple ID) you use for your Apple Account while retaining everything in your account.
If you have an iPhone or iPad:
1. Go to Settings, then tap your name.
2. Tap Sign In & Security.
3. Next to Email & Phone Numbers, tap Edit.
4. Tap the red Remove button next to the email address, then tap Delete.
5. Tap Choose Another Email Address and enter your device passcode if asked.
6. Enter the new email address, then tap Continue.
7. Enter the verification code that was sent to your new email address. Read this support article to learn what to do if you didn’t get the email —> If you didn’t receive your verification or reset email - Apple Support
8. If you encounter issues with certain services after doing this, this outdated support article may still have useful suggestions --> What to do after you change your Apple ID or password - Apple Support
Refer to the support article Change your Apple Account primary email address - Apple Support :
- If you don’t have an iPhone or iPad, the support article says to, “go to https://account.apple.com and sign in. Then follow the onscreen instructions to change the email address associated with your Apple ID.”
- For general information, and other issues such as changes or addresses not being accepted.
- Note a recent change to the article where you can now change the email address without having to have access to your old email account.
