Provide support/advice on Office365 Portal

I have a client who migrated accounts from Google to Office365. They wanted to reduce licence costs, and so two accounts in Google were merged into one Office365 account, with an alias address for the second. Whilst having one inbox isn't an issue for both email addresses - they're unable to send emails from the alias address. I need someone who's able to assist me in being able to setup the user's account so that they're able to send as the alias account, as well as the primary. If this isn't possible - then I will need someone who can assist me in splitting the existing mailbox between two accounts. I have admin access to the Office365 portal Users are accessing emails via Outlook OWA