too vs besides

too and besides both are adverbs.

Word	Noun	Adjective	Verb	Adverb
too	No	No	No	Yes
besides	No	No	No	Yes

As adverbs, too and besides are synonyms defined as:

Other synonyms of too include also, as well, likewise.

too (adverb)	besides (adverb)
to a degree exceeding normal or proper limits	making an additional point; anyway
in addition	in addition