split up vs step-up

split up and step-up both are nouns.

Word	Noun	Adjective	Verb	Adverb
split up	Yes	No	Yes	No
step-up	Yes	No	No	No

As nouns, step-up is a hypernym of split up; that is, step-up is a word with a broader meaning than split up:

split up: an increase in the number of outstanding shares of a corporation without changing the shareholders' equity
step-up: the act of increasing something

Other hypernyms of split up include increase.

split up (noun)	step-up (noun)
an increase in the number of outstanding shares of a corporation without changing the shareholders' equity	the act of increasing something

split up (verb)	step-up (verb)
become separated into pieces or fragments
get a divorce; formally terminate a marriage
discontinue an association or relation; go different ways
separate into parts or portions