Two User mailboxes added in outlook but only one mailbox sent items is showing emails for both

Hi,
As per your description, I believe that both mailboxes are having full delegation against each other..
Please correct me, if I understood wrongly....

Is it a different Domain or same Domain????
mailbox @domainA
mailbox @domainB

Whenever you sent an email.. Are you choosing the correct mailbox in OUTLOOK before sending emails?

I think you are not choosing the correct mailbox while sending emails that why it is going to MailboxA sent items.

Please ask your Exchange Admin to run the below command, hopefully it will resolve your issues:

set-mailbox -Identity "mailbox@domainB" -MessageCopyForSentAsEnabled $True

https://docs.microsoft.com/en-us/exchange/recipients-in-exchange-online/manage-user-mailboxes/automatically-save-sent-items-in-delegator-s-mailbox


Thank you,

Regards,
MD