Interesting scenario. I’d be interesting in also knowing if your client or the niche company is the ‘dominate’ acquisition. Nonetheless, I would recommend a few ideas:
– allow some job shadowing… this will give each side an opportunity to learn about each other:challenges, strengths and even culture. This should be a well thought out plan and execution with some specific goals. The duration of this may vary by functional areas.
– virtual WFH is a great opport. to still get folks together and I suggest that in addition to the normal integration work, you should organize some social events.
From an integration perspective, I think you (leadership) needs to decide what is the new culture, pretty quickly, so that you establish alignment early. To determine that, again, depending on the companies/industries, you may want to get feedback from both, especially if after they have walked in each other’s shoes, they have a new appreciation for their cultural differences.