I have two Microsoft accounts on different email addresses

Combining or merging two Microsoft accounts into one is not supported.

What can be done instead:

1. Decide which account to keep
   Since one subscription is to be kept and the other dropped, continue using the Microsoft account that has the active Microsoft 365 subscription that should remain.
2. Use both accounts side by side (optional)
   Even though accounts cannot be merged, they can be used together:
   • Add both accounts to Outlook so email from both addresses can be accessed in one place. See Add accounts to Outlook in the combining-accounts article.
   • Use both OneDrive for work/school and OneDrive personal on the same device; storage stays separate but files can be copied between them.
   • Set up multiple profiles in Microsoft Edge so each account has its own browser profile.
3. Cancel the extra subscription
   • Go to https://account.live.com and sign in with the Microsoft account whose subscription should be cancelled.
   • Go to Subscriptions.
   • Select Manage for the plan that should be cancelled and follow the prompts to cancel.
     Cancelling a subscription does not delete the Microsoft account or its email address; it only stops the paid plan.
4. (Optional) Close the unneeded Microsoft account
   If the second account is no longer needed and there is no important data or purchases on it, it can be closed after cancelling any subscriptions. Follow the steps in How to close your Microsoft account from the referenced Q&A.
5. If one account uses the same email domain and causes prompts
   If there is ever a “Which account do you want to use?” prompt (work/school vs personal) on the same email address, that is because two different account types share the same sign-in. In that case, the personal account can be renamed by adding a new alias and making it primary under Manage how you sign in.
6. To speak with Microsoft about billing
   For subscription and billing help (including cancelling one of the subscriptions or refund questions), contact Microsoft Support:
   • Open the Get Help app or go to the Microsoft Support site.
   • Type a brief description (for example, “billing – cancel extra Microsoft 365 subscription”).
   • Choose Get Help → Contact support.
   • Under Products & Services, select Microsoft 365 & Office.
   • Under Category, choose Manage my subscription.
   • Choose Chat with a support agent or Call me back and provide a phone number so a support agent can call.

This will allow keeping one paid subscription, optionally keeping access to both email addresses, and getting direct assistance from a live Microsoft support agent for billing.

References:

• Combining Microsoft accounts
• Which account do you want to use?
• I am paying for two subscriptions - Microsoft Q&A
• I realize I have 2 different accounts. How do I combine the 2? - Microsoft Q&A
• How do I get a live person at Microsoft.? - Microsoft Q&A