@Ch213I don't think you cant change it to a different bank account for one or the other as per these T&C's
Your Paired iPhone Airtime Plan and Watch Airtime Plan must be on the same billing account. You need a Watch Airtime Plan for each Watch that you want to pair with an iPhone. You can only pair one Apple Watch per iPhone.
I know from my own account, i cant change any of the numbers to have different Direct Debit to the others.
- Xperia 1V - o2 and Spusu
- Pixel 8 Pro - o2 and Vodafone UK