Managing cultural integration

You could think about a cultural assessment – it would depend on the size of the two companies merging on what levels you want to conduct such an assessment and how much time it would take. After the assessment, use a focus group to define the ultimate culture you want to have at the end, and then work with a group specialized in change management, cultural awareness and communication on how the roll-out of the results could look like. I strongly believe consistent and transparent communication is key in case it means the culture is shifting at one of the two companies.
Another best practice could be developing a shared vision and shared core values, especially to highlight the already existing overlap – always good to stress the commonalities.