Communication is key, and the right message has to be shared way before the merger is complete.
Oftentimes employees hear in the press that their company is a target for an acquisition, and yet within the company, the management is not saying anything (or not saying much) about it. This gap in communication creates space for fear, conspiracy theories, etc. It is in fact an ideal opportunity for the management to begin to sell the story of why the merger would be good for the company and for the employees, rather than waiting until the deal is completed. At this early stage, discussions about possible synergies, culture alignment, etc can already start happening. Also, it should be clear from the due diligence where culture misalignments exist. The management should be addressing these by this stage.
