in tandem vs tandem

in tandem is not a noun while tandem is a noun.

Word	Noun	Adjective	Verb	Adverb
in tandem	No	No	No	Yes
tandem	Yes	No	No	Yes

As adverbs, in tandem and tandem are synonyms defined as:

in tandem (noun)	tandem (noun)
	a bicycle with two sets of pedals and two seats
	an arrangement of two or more objects or persons one behind another

in tandem (adverb)	tandem (adverb)
one behind the other	one behind the other