subbing vs substituting

subbing and substituting both are nouns.

Word	Noun	Adjective	Verb	Adverb
subbing	Yes	No	No	No
substituting	Yes	No	No	No

As nouns, subbing and substituting are synonyms defined as:

subbing and substituting: working as a substitute for someone who is ill or on leave of absence

subbing (noun)	substituting (noun)
working as a substitute for someone who is ill or on leave of absence	working as a substitute for someone who is ill or on leave of absence