I should of added that "Conn" in the old naval sense was used in TOS. Kirk would tell an officer on the bridge that the person "has the conn" which gives them control of the ship. This role has a much higher authority and responsibility than TNG's "Conn" for a flight control/helm officer.
Hmmm, I wonder if in TOS if it was normal for first officers to have dual roles. We'll never know since TOS only presented us with one ship's crew to follow. But, hypothetically, if Spock wasn't the first officer and it was not a dual role then where would the first officer sit? Perhaps the station between Spock and Uhura?
Spock and Uhura's stations are right next to each other, so I don't understand, unless you are suggesting that one shot in "The Doomsday Machine" is the actual layout, or that this hypothetical situation had the science station in a different place.
I thinks he just stands around near the captain's chair getting in the way of the yeoman bringing coffee and reports to the captain. When not getting under foot, he's probably walking the ship chatting with lazy crewmen.
After Spock arrives and takes over as the science officer, what does Decker really do on the ship as XO? As near as I can see, it is to argue against every command order that Kirk makes

, then sit in his chair whenever Kirk leaves the bridge.
I was going to compare this situation to Riker, but it seems you've already headed in that direction

Seriously, though, as funny as that sounds, it worked on TNG for the drama; even though Riker has a chair it is kind of the same thing: his official role is to provide alternative plans to the captain and also do some personnel assignment work. This is not a criticism of Riker or his role on the show at all. It seems like this was in Roddenberry's idea for a new show.
What I was suggesting is that one of the roles in the pit is the "typical" first officer location and job for the person and would not be considered "dual role" on the hypothetical "typical ship."
There is a pattern: Number One sat there first; Kelso has some kind of important ship operations job; Sulu is one of the command crew and sits there; again in TMP Sulu is one of the main people to take command in the absence of the Captain and sits there; Data is Chief of Operations (and if the Galaxy Class did not make a separate chair for the first officer, the ranking officer below the captain would be sitting there, it seems).
The science station being a different place in TMP may be significant, as it is directly behind the captain, and has details not found on the other stations. Decker would be a bit of an unusual situation since there truly is an extra officer once Spock arrives. (Possibly an extra "staff" officer based on prior thread's discussions

) It seems as if the science station on this version of the Bridge is set up to do many things without the captain monitoring it, since he would have to look behind him and away from the screen. So perhaps the science station was to have possibly special significance, or possible less story significance in this new design. Does the Phase II version of the set, albeit incomplete, still have the stations moved to new locations?