Have 2 countries ever merged?
 

Countries take over other countries and countries split up but do they ever decide to just get together? Seems weird that they wouldn't.

Bosnia and Hercegovina i think?

Don't know to be honest. Good thread.

probably not, banana countries are much better

If by countries you mean nations, as in a large body of people associated with a particular territory then yes. It's been going throughout history. Just after WWI the Czechs and Slovaks of the former Austro-Hungarian Empire merged to form Czechoslovakia. Well, you know the rest of the story.

East and West Germany may also fit what you're asking about.