There's another TOS Harrison. The entry below is from the Lexicon of my trusty 1976 edition of the "Star Trek Concordance".
"Harrison, Dr. (John Bellah): Young pathologist who catches the Psi 2000 virus; Spock sees him in a passageway, painting "Love Mankind" and "Sinner Repent" on the bulkheads and laughing hysterically. McCoy later calls Dr. Harrison on the intercom and gets nothing but laughter."
The entry for Veto's Harrison appears uncredited to an actor as:
"Harrison, Technician 1/C: Crewman on the bridge when Khan Noonian Singh takes over and nearly suffocates the entire bridge crew."
For what it's worth, there's a third Harrison listed. William B. Harrison (uncredited to an actor) who was the last of Captain Merik's men to die in "Bread and Circuses".