two-seater vs roadster

two-seater and roadster both are nouns.

Word	Noun	Adjective	Verb	Adverb
two-seater	Yes	No	No	No
roadster	Yes	No	No	No

As nouns, two-seater and roadster are synonyms defined as:

two-seater and roadster: an open automobile having a front seat and a rumble seat

Other synonyms of two-seater include runabout.

two-seater (noun)	roadster (noun)
an open automobile having a front seat and a rumble seat	a small lightweight carriage; drawn by a single horse
	an open automobile having a front seat and a rumble seat