One Apple ID for two numbers but would like to shift one number to a new Apple ID

Hi caterina43,

There are a few considerations to take when creating a separate Apple ID for someone who has been sharing with someone else:

1. Make sure that any data your mum syncs with iCloud that she wants access to once she has her own Apple ID has been saved or archived, following the steps in Archive or make copies of the information you store in iCloud - Apple Support. As there is no way to automatically transfer content from one Apple ID to another, this will be how she ensures she does not lose any important data.
2. Media downloaded from the various stores (iTunes Store, App Store, Books Store) cannot be transferred from one Apple ID to another. However, you can use Family Share to share purchases with up to five family members. More information on this can be found here: What is Family Sharing? - Apple Support
3. You will want to ensure your mum's phone number is no longer associated with your dad's Apple ID if they use iMessage or FaceTime. This will prevent her from getting calls on her device when someone calls your dad: Add or remove your phone number in Messages or FaceTime - Apple Support

Once you've done this, you should be able to set your mum's new Apple ID up with the steps here: How to create a new Apple ID - Apple Support.

Have a wonderful day!

caterina43, You're welcome! Glad we could help. Thanks for using the Apple Support Communities. Cheers!