fellow worker vs workfellow

fellow worker and workfellow both are nouns.

Word	Noun	Adjective	Verb	Adverb
fellow worker	Yes	No	No	No
workfellow	Yes	No	No	No

As nouns, fellow worker and workfellow are synonyms defined as:

Other synonyms of fellow worker include co-worker, colleague.

fellow worker (noun)	workfellow (noun)
an associate that one works with	an associate that one works with