apple music/billing

I have a question relating to billing for Apple Music. I have 2 phones on one Verizon account, and each device has its own Apple ID. I am currently paying an individual monthly subscription (9.99) on device number 1 and have been doing so for several months. I recently added a 3 month trial to device number 2 (to be billed 14.99/month since I chose the family option). I was thinking I could have both accounts (separate apple id's) and be billed 14.99/month for both. I'm afraid that once the 3 month trial is over on March 22, that Apple will charge me 9.99/mo for device 1 and then 14.99/month for device 2. I'm not sure how to merge the two or if the billing for Apple Music is associated with the credit card number or the Apple ID?

Or should I cancel my free trial on the second device and switch the subscription on the first device to a "family" option? I'm not sure how this works...