untier vs unfastener

untier and unfastener both are nouns.

Word	Noun	Adjective	Verb	Adverb
untier	Yes	No	No	No
unfastener	Yes	No	No	No

As nouns, untier and unfastener are synonyms defined as:

Other synonyms of untier include opener, undoer.

untier (noun)	unfastener (noun)
a person who unfastens or unwraps or opens	a person who unfastens or unwraps or opens