united vs joined

united and joined both are adjectives.

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

As adjectives, united and joined are synonyms defined as:

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

united (adjective)	joined (adjective)
characterized by unity; being or joined into a single entity	connected by a link, as railway cars or trailer trucks
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