overflow vs spill over

overflow is a noun but spill over is not a noun.

Word	Noun	Adjective	Verb	Adverb
overflow	Yes	No	Yes	No
spill over	No	No	Yes	No

As verbs, overflow and spill over are synonyms defined as:

Other synonyms of overflow include bubble over.

overflow (noun)	spill over (noun)
a large flow
the occurrence of surplus liquid (as water) exceeding the limit or capacity

overflow (verb)	spill over (verb)
overflow with a certain feeling	overflow with a certain feeling
flow or run over (a limit or brim)	be disgorged