Yeah, I double checked, and Tom Nellis was first officer and the navigator for Antares
. Number One was at the helm and definitely first officer, too.
Frankly, in TOS "First Officer" just means the second-in-command, and the position may not come with any specific duties the way an executive officer position would. So Spock pulling double duty wouldn't be that impressive, and the scant evidence in the show points to all First Officers having a useful posting, too.
Therefore Spock probably isn't valuable just because he can do both the Science Officer and First Officer jobs simultaneously. So why can he get away with hijacking a starship without a real