roadster vs two-seater

roadster and two-seater both are nouns.

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

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

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

Other synonyms of roadster include runabout.

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