It’s a bit confusing. From wiki, it’s basically something like this:
In 1945 the ‘Federal People’s Republic of Yugoslavia’ was formed after WW2.
Then in 1963 the country became known as the ‘Socialist Federal Republic of Yugoslavia’ (SFR Yugoslavia).
Then in 1992 that country dissolved and became known as simply the ‘Federal Republic of Yugoslavia’ (FR Yugoslavia). That final third ‘rendition’ of Yugoslavia was composed of just two states - Montenegro and Serbia.
In 2003 the country of ‘FR Yugoslavia’ transformed from a Federal Republic to a Political Union between the two states and the country became known as ‘Serbia and Montenegro’.
Then in 2006 Montenegro succeeded from the union and became independent.
So basically it’s not two countries joining together, ‘Serbia and Montenegro’ was the name of the country that was created out of Yugoslavia for a few years until the two states split and became independent nations!
In 1945 the ‘Federal People’s Republic of Yugoslavia’ was formed after WW2.
Then in 1963 the country became known as the ‘Socialist Federal Republic of Yugoslavia’ (SFR Yugoslavia).
Then in 1992 that country dissolved and became known as simply the ‘Federal Republic of Yugoslavia’ (FR Yugoslavia). That final third ‘rendition’ of Yugoslavia was composed of just two states - Montenegro and Serbia.
In 2003 the country of ‘FR Yugoslavia’ transformed from a Federal Republic to a Political Union between the two states and the country became known as ‘Serbia and Montenegro’.
Then in 2006 Montenegro succeeded from the union and became independent.
So basically it’s not two countries joining together, ‘Serbia and Montenegro’ was the name of the country that was created out of Yugoslavia for a few years until the two states split and became independent nations!