split up vs break apart

split up is a noun but break apart is not a noun.

Word	Noun	Adjective	Verb	Adverb
split up	Yes	No	Yes	No
break apart	No	No	Yes	No

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

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

split up (verb)	break apart (verb)
become separated into pieces or fragments	break up or separate
get a divorce; formally terminate a marriage	break violently or noisily; smash
discontinue an association or relation; go different ways	take apart into its constituent pieces
separate into parts or portions