twopenny-halfpenny vs two-a-penny

twopenny-halfpenny and two-a-penny both are adjectives.

Word	Noun	Adjective	Verb	Adverb
twopenny-halfpenny	No	Yes	No	No
two-a-penny	No	Yes	No	No

As adjectives, twopenny-halfpenny and two-a-penny are synonyms defined as:

Other synonyms of twopenny-halfpenny include sixpenny, threepenny, tuppeny, twopenny.

twopenny-halfpenny (adjective)	two-a-penny (adjective)
of trifling worth	of trifling worth