Find my app, people

If they are sharing an Apple ID then there's no way to separate devices belonging to one or the other.

Also, the phone number has nothing to do with Find My and tracking a phone.

They should not be sharing an Apple ID at all.

If you are trying to track using the People option in Find My you will only get the location for whatever device has been set to share the location from. To track a specific device, you would need to log in with their Apple ID at https://appleid.apple.com and select the specific device from the list in the Find My app there.

I don't think the location feature goes by telephone number, it goes by Apple ID and where the activity is. Since they are both sharing an ID (not what Apple recommends --> Make sure that each family member has a unique Apple ID - Apple Support) they are seen by Apple as being the same person.