I haven’t personally tried this approach before, but it sounds like a very good idea. Bringing the two companies together on a smaller project first would give you a practical way to observe the cultural differences in action, rather than relying only on assumptions. It can help reveal how each team works, makes decisions, communicates, manages deadlines, and responds to pressure.
This kind of pilot project could also create a safer space to identify and address friction before the full merger happens. If the work ethic and pace are very different, it is better to surface those issues early with a small group than discover them after the businesses are already combined. In that sense, your approach acts like a form of cultural due diligence and could help both owners make a more informed decision about whether the merger can truly succeed.