Migrating from Google Workspace to Microsoft 365 with to have two separate Mailboxes per User + SSO

Context and Requirements:

My company (abc.com) is being acquired by another org (xyz.com). We’re currently on Google Workspace (G Suite) for abc.com email and want to migrate everything (email, calendar, contacts) into Microsoft 365 — specifically into the xyz.com tenant. After migration, each user must have two fully separate mailboxes: Mailbox A: email address removed for privacy reasons (data migrated from G Suite)

Mailbox B: email address removed for privacy reasons (new mailbox in the xyz.com tenant)

We need true single sign-on (SSO) so users can log in once and access both mailboxes (not just alias addresses) within the same tenant. We want to minimize complexity. FYI we will set up abc.com as an Accepted Domain in xyz.com Microsoft Exchange tenant.

What I’ve Investigated So Far:

Migration Tools:

The Exchange Admin Center has a built-in G Suite migration wizard, but it mainly migrates mail to one mailbox.

Third-party tools like BitTitan MigrationWiz can also handle G Suite → M365 migrations. 

Two-Mailbox Setup Approaches: One licensed mailbox + one shared mailbox: For example, the user’s main mailbox would be email address removed for privacy reasons (fully licensed with migrated data), while email address removed for privacy reasons is a shared mailbox to keep it separate. The email address removed for privacy reasons account gets Full Access and Send As permissions on the xyz.com mailbox.

Pros: Single sign-on, one set of creds, minimal extra licensing. The data stays separate (two distinct mail stores).

Cons: Shared mailboxes are limited to 50GB if unlicensed, and some compliance features may require a license. 

Two fully licensed user mailboxes (two Azure AD user accounts, e.g. email address removed for privacy reasons and email address removed for privacy reasons): Each mailbox is completely independent, but typically requires two sign-ins unless there’s advanced identity federation to unify credentials. Also doubles the licensing.

Pros: Full feature set in both mailboxes, no shared mailbox constraints.

Cons: Additional license costs, and not a straightforward “one-click” SSO for both.

Questions 

Which approach is best for maintaining two discrete mailboxes per user without forcing them to manage multiple logins?

Are there any best practices for migrating from Google Workspace into Exchange 365(specific tenaant) maintaining the same email address?

Have you run into compliance or eDiscovery issues with a shared mailbox approach?

Any caveats or special steps needed to ensure calendars and contacts migrate correctly from Google Workspace into the new environment?