Microsoft needs to explain this better, so users will understand:
1- All software and services that you purchase from Microsoft - including subscriptions, apps and OneDrive storage - go through your Microsoft account.
2- In order to make a purchase, a Microsoft account needs to have at least one stored payment method.
3- The Microsoft account that makes the purchase with its stored payment method is the Microsoft account that owns the subscription, app or storage.
4- To use the subscription, app or storage that your Microsoft account purchased, you have to enter the username and password of that Microsoft account.
In your case, the Microsoft account that purchased an additional 1TB of storage is the Microsoft account which received that storage, so you will find that 1TB of storage in the OneDrive belonging to that Microsoft account. To use that 1TB of space, you must sign in with the username and password of the Microsoft account that purchased the space.