I manage an office 365 A1 for a school, used for Teams for adults and students, configured with one domain name, we call that domain : X (firstname.lastname@X)
I decided to quit our old professionnal mailboxes hoster to exchange online, our boxes have another domain name, we call that domain : Y (firstname.lastname@Y), used for our adults only. We want to keep that domain for institutionnal emails.
All users exists on our O365 (exchange plan), i just add Y in O365 and moved dns records.
At this moment, adults have 1 exchange mailbox with 2 addresses (X + Y), both working great in reception. I set (for each adult) the Y address as default login-address (they must login to Teams with the Y address now). Our tenant keep the X address as default domain, especially for non-adults accounts.
When adults send emails using webmail (not tested in outlook client), mails are sent using the X address, even if the from field (ellipsis button) show the Y address!