Looking for best strategy to integrate an aquired company into 365

Question

We (Company 1) recently acquired Company 2. We have a plan to fully merge them into our systems by the end of the year.

Company 1
-Azure AD sync with several DC's
-Hybrid Exchange Environment

Company 2
-No Azure sync with 2 isolated Forests (company1a.com & company1b.com)
-Full 365 email (no on-prem exchange)

We want to eventually convert all users to be "company 1" employees, everything down to their email will be company1.com (retaining control of their domain for historical purposes). I will have full control of their domains soon, and I am curious on the best way to go about this. Should I replicate our setup on theirs, as in setup Azure sync and then merge them into our tenant? Or merge the current 365 setup into ours, move the DC's over the old school way, then sync them using our setup? Is there any benefit one way or the other?

Answer 1

A lot of variables can come into play. To start working out a strategy on headlines, consult the PDF linked one this page:
https://learn.microsoft.com/microsoft-365/enterprise/microsoft-365-tenant-to-tenant-migrations?view=o365-worldwide&WT.mc_id=M365-MVP-5000284