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Based on the information provided in the context, to add another owner to an Azure subscription, the current subscription owner needs to delegate access management for an individual subscription by assigning the Owner role to a user at the resource scope of the subscription. If you want to give limited access, select a different role. For detailed steps, see Assign Azure roles using the Azure portal. Alternatively, the current subscription owner can add a user as a billing administrator to an Enterprise Agreement or a Microsoft Customer Agreement.
If the alt account is already the subscription owner and is unable to add another owner, it may be necessary to check if the subscription type supports billing ownership transfer, as not all subscription types support it. Additionally, if the subscription is part of an Enterprise Agreement or a Microsoft Customer Agreement, the alt account may need to have the appropriate billing profile role to manage all reservation purchases made using the billing profile.
Note: This response is partially based on Microsoft Q&A AI Assist and posted after manually validating the accuracy of the response and making necessary changes as appropriate.