I’m not sure if I understand you correctly, but I think you mean the employees will recieve emails @domainA.com, as well as @domainB.com.
If so, for domainB.com users, you can create contact accounts with SMTP of domainB.com and have domainA Exchange box forward it to domainB’s EXCH box.
Alternatively you can add another Recipient Policy in domain A’s EXCH box with SMTP of domain B (@domainB.com). This requires you create AD accounts for domain B users.
As for public MX records you can keep them the same as they are now.