split up vs fall apart

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

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

As verbs, split up and fall apart are synonyms defined as:

Other synonyms of split up include break, come apart, separate.

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

split up (verb)	fall apart (verb)
become separated into pieces or fragments	become separated into pieces or fragments
get a divorce; formally terminate a marriage	break or fall apart into fragments
discontinue an association or relation; go different ways	go to pieces
separate into parts or portions	lose one's emotional or mental composure