split up vs disunite

split up is a noun but disunite is not a noun.

Word	Noun	Adjective	Verb	Adverb
split up	Yes	No	Yes	No
disunite	No	No	Yes	No

As verbs, disunite is a hyponym of split up; that is, disunite is a word with a more specific, narrower meaning than split up:

split up (noun)	disunite (noun)
an increase in the number of outstanding shares of a corporation without changing the shareholders' equity

split up (verb)	disunite (verb)
become separated into pieces or fragments	force, take, or pull apart
get a divorce; formally terminate a marriage	part; cease or break association with
discontinue an association or relation; go different ways
separate into parts or portions