Which of the following terms is the correct one for "2 in 1"?
Zwei in Eins
Zwei in Einem
As a single expression, it's "Zwei in Einem", because "in" is with dative when it's a place, not a direction. However, you could say "Zwei in Ein(e)s" if you wanted to put two of something into one (e.g. two eggs into one glass).
"Wollen Sie jedes Ei in ein separates Glas?" - "Nein, werfen Sie zwei in eines."
(Please, don't ask why I am talking about eggs in a glass.)
In marketing, it is, however, always written as "2 in 1" (similar to "2 für 1"), and often colloquially pronounced as "zwei in eins", just because this is how one would literally read it out, and maybe also because of the similarity to "2 für 1", where it is accusative.
Using "in" as a preposition for defining a spatial realtionship needs a dative object that may be further specified by using a numeral.
Obviously the object was omitted in the example given but still we need the numeral to follow the case of the object (i.e. dative):
Zwei in einem - (if the object is masculine or neuter like e.g.) - Sack
Zwei in einer - (if the object is female like e.g.) - Packung
In modern marketing or commercials these finer points of grammar are sometimes thought to lead away from the message. Therefore we may hear grammatically incorrect slogans like it would be here with "zwei in eins".