For IMAP accounts, you cannot change where sent messages are stored. More information, please refer to:Change where sent email messages are saved - Microsoft Support.
If you want to sync the two sent folders, you can move the emails of "Sent Items" to the "Sent" folder. To move a folder, follow these steps: Select Sent Items> select Move > other folders under the Home tab> select the Sender folder under the corresponding account.
Hope this helps!
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.