harmoniser vs go-between

harmoniser and go-between both are nouns.

Word	Noun	Adjective	Verb	Adverb
harmoniser	Yes	No	No	No
go-between	Yes	No	No	No

As nouns, go-between is a hypernym of harmoniser; that is, go-between is a word with a broader meaning than harmoniser:

harmoniser: a mediator who brings one thing into harmonious agreement with another
go-between: a negotiator who acts as a link between parties

Other hypernyms of harmoniser include intercessor, intermediary, intermediator, mediator.

harmoniser (noun)	go-between (noun)
a mediator who brings one thing into harmonious agreement with another	a negotiator who acts as a link between parties
a musician who sings or plays in harmony