Hi @Andrea Vironda ,
What's the type of your account? POP, IMAP or Exchange?
What's the version of your Outlook? (File > Office Account > About Outlook). Please make sure you have upgraded your Outlook on both devices to the latest version.
Before operations, it is suggested for you to back up your mailbox items on both devices to avoid loss. As I know, for IMAP and Exchange email accounts, emails are all synced with your mail server. So, in order to further confirm your issue, please try to log in to your web mail and modify your folder structure as you wish. Try sending emails from the web mail to confirm which is the correct Sent item folder, move emails in another Sent item folder to this folder and remove the another Sent item folder.
Then, on both devices, please try creating and using a new Outlook profile via Control Panel > Mail > Show profile and re-add your email account to see if the issue could be resolved.
If this is not your case, please try to provide me with more information related to your issue.
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