split up vs break open

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

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

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

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

split up (verb)	break open (verb)
become separated into pieces or fragments	erupt or intensify suddenly
get a divorce; formally terminate a marriage	come open suddenly and violently, as if from internal pressure
discontinue an association or relation; go different ways	open with force
separate into parts or portions