There's a lot of talk about how this is supposedly a good idea for both sides, but here's what actually happens:
The people working in the "lesser" part take a very close look at who the new management is, and what their market chances are. The most likely outcome here is "meh".
At that point, most career-hungry people who have contacts in the other side of the company start extending feelers, because it's better to be in a growth area than in a steady ship if you want to have quick career growth. The few who've also got contacts outside the industry weigh the rest of the industry for their prospects.
Meanwhile, middle management isn't stupid and knows this is happening. Large turf wars break out, everybody trying to secure the most interesting projects for their teams so they can attract the best remaining people. The resulting office politics drive most of the remaining people who have options outside to leave as well, because it's a cesspit.
At this point you have created a solidly mediocre company. It'll likely plod on for a long time, on a slow downward slope. Every calculates what comes first, implosion or retirement, and chooses accordingly.
Life is, for lack of a better word, solidly grey.
But sure, on paper it's a great opportunity for both sides.