Both your iPad and iPhone should be signed-in to iCloud with the same AppleID. When both are signed-in, you won’t need to use family sharing.
If you intentionally wish to use two different AppleID accounts, only then might you wish to consider Family Sharing with your second AppleID account - but this will lead to multiple complications that you’ll likely wish to avoid.
If you do wish to change from using two AppleID accounts - and use only one - decide which account you’ll use moving forward. You’d be wise to choose the AppleID account that you’ve used to purchase Apps - as these purchased Apps are forever tied to the AppleID account from which they were purchased.
Having chosen the account that your wish to use - you’ll need to change the signed-in account on the device that you wish to change. First ensure that you have disabled the Activation Lock that will otherwise cause difficulties in the future. You can disable the Activation Lock by disabling the Find My iPad service from iPad settings before trying to use a different AppleID account:
Settings > [Your Name / AppleID] > Find My > Find My iPad - set to OFF
Having disabled the Activation lock, you can now sign-out from your old AppleID:
Settings > [Your Name / AppleID] > Sign Out
Presumably you’ll want to keep any iCloud data that is locally stored on your iPad. If so, accept the opportunity to keep a local copy of your data (e.g., Calendar, Contacts, Notes, Keychain Passwords etc.) - as otherwise this data will be wiped from your iPad as you sign-out from iCloud.
Also ensure that you have signed-out from Messages and FaceTime:
- Settings > Messages > Send and Receive > [AppleID] - scroll to the bottom where you will find your signed-in AppleID highlighted in blue text; tap and select Sign Out
- Settings > FaceTime > [AppleID] - again, scroll down the page to find your signed-in AppleID highlighted in blue text; tap and select Sign Out
After signing out, a restart of your iPad is recommended:
Restart your iPad - Apple Support
After restarting, you can sign-in to the wanted AppleID account from iPad settings - and also sign-in to Messages and FaceTime if necessary. Any iCloud data that you saved to your iPad during the previous sign-out will be merged with your wanted AppleID account.
