In my experience, holding townhalls with both executive leaders present to communicate goals, impacts, timeline and rationale behind the merger.
Soliciting feedback from both companies through surveys, open door policies or email will make the two companies feel valued and help you with lessons learned.
Orientation and Onboarding activities with a white glove approach to help guide all levels of EE.
Being flexible and having the ability to adjust if things are headed in the wrong direction.