Not sure if an acquired company can decide to opt out after Post Close. Terms and conditions specified in merger dictate the future of acquired company which gets discussed prior to the finalization of the deal.
In event of merger being part of portfolio expansion strategy, it makes sense for the two businesses to operate separately. But if both of the companies own similar line of products and operate in similar LoB, then it will hurt operating as two different entities as they face the risk of cannibalization of their products.
In high-tech, it is common practice for acquired company to operate separately for specific period until all the OUs or BUs integrate.
