combination vs uniting

combination and uniting both are nouns.

Word	Noun	Adjective	Verb	Adverb
combination	Yes	No	No	No
uniting	Yes	No	No	No

As nouns, uniting is a hyponym of combination; that is, uniting is a word with a more specific, narrower meaning than combination:

combination (noun)	uniting (noun)
the act of combining things to form a new whole	the act of making or becoming a single unit
the act of arranging elements into specified groups without regard to order	the combination of two or more commercial companies
a collection of things that have been combined; an assemblage of separate parts or qualities
a group of people (often temporary) having a common purpose
a sequence of numbers or letters that opens a combination lock
an alliance of people or corporations or countries for a special purpose (formerly to achieve some antisocial end but now for general political or economic purposes)
a coordinated sequence of chess moves