joined vs united

joined and united both are adjectives.

Word	Noun	Adjective	Verb	Adverb
joined	No	Yes	No	No
united	No	Yes	No	No

As adjectives, joined and united are synonyms defined as:

joined and united: of or relating to two people who are married to each other

joined (adjective)	united (adjective)
connected by a link, as railway cars or trailer trucks	characterized by unity; being or joined into a single entity
of or relating to two people who are married to each other	of or relating to two people who are married to each other
	involving the joint activity of two or more