incompetence vs incompetency

incompetence and incompetency both are nouns.

Word	Noun	Adjective	Verb	Adverb
incompetence	Yes	No	No	No
incompetency	Yes	No	No	No

As nouns, incompetence and incompetency are synonyms defined as:

incompetence and incompetency: lack of physical or intellectual ability or qualifications

incompetence (noun)	incompetency (noun)
lack of physical or intellectual ability or qualifications	lack of physical or intellectual ability or qualifications
inability of a part or organ to function properly