They could be two separate subscriptions, particularly if you see you are being billed twice. The other possibility is you set up Family Sharing between the two accounts, in which case only one account is paying for the subscription. Have a look at the purchase history for each account.
Read this support article for directions about how to see your purchase history in the App Store or iTunes Store [mobile device, computer, or web] --> View your purchase history for the App Store and other Apple media services - Apple Support
- If Family Sharing is enabled, you can see your family members’ purchases. Select the Apple Account menu and choose a family member.