I think language and culture have usually been the major impediments in many merged countries. The most frequent solution was to impose the dominant language (or dialect) and culture on the junior partner or conquered area. In many cases, the local population continued to speak their own language (or dialect), but were forced to conduct all governmental business in the dominant language. Language is still causing conflict in currently existing countries like Belgium and Spain. Puerto Rico is unlikely to become a US state because of the linguistic issue (as well as the economic conditions).
Imposing a single currency has been done many times throughout history. The easiest on the public is probably to declare an official exchange rate between the existing currencies and either phasing out one in favor of the other, or replacing both with a new currency gradually. More disruptive is to abruptly impose the dominant currency and no longer recognizing the other – usually the result of a hostile takeover. The US Dollar is the official currency in several independent countries around the world. And then there’s the Euro…
;–)
Legal systems and government institutions also need to be combined. The ‘one country, two systems’ policy doesn’t seem to be working in China these days. But there were several cases in Europe where one sovereign ruled over two kingdoms that officially remained separate.