Unfortunately, there is no way to merge the two Stock accounts, however, we can close one of your accounts and can add the remaining assets or images(of the closed account) in the account which is active (as an exception).
Additionally, I would like to let you know that you forfeit any unused licenses upon cancellation of your subscription, so you'd need to use them prior to cancelling.
For more information please see Common Questions about plans, purchasing, and availability, Adobe Stock
Hope this information helps!
Feel free to update this thread in case of any additional questions.
Regards
Twarita