...But I don't think they are done this way at all (since Morrowind). My memory of Oblivion is that EVERY interior that I edited was one full model, not little kit pieces, and I checked a fair few, from Skingrad, Leyawiin and Chorrol mostly, including very large ones. They probably did design the exteriors and interiors concurrently, but it's still just sloppy. They should have 1) Checked on each others work as they went along to make sure they matched, or 2) Made a basic building shape design sketch for the other team person to copy as well as details on what parts would be made of what.
I'm not disagreeing with your point about coordination, but many ARE built with kit pieces because I’ve edited some of the player home interiors via the console. (Namely Breezehome and Proudspire Manor) For example, the basemant level in Proudspire Manor is all kit pieces. (The Blue Palace and many other Solitude interiors use the same kit as well.) But 4 different pieces make up the enchanting area, 4 more the alchemy area and so on. In fact, the wall that blocks off the junk area where the static crates, barrels, bedroll and rugs are can be removed to expand that room without even adding another piece in. I removed it, added a lot of storage and a bed to that room so I can just pop in the lower side entrance, dump my gear and head on back out in a snap. So, I do believe that many are built this way, but I also have the suspicion that some are one large piece as you said as well. (I hated not being able to design my own rooms in Oblivion so I know well of what you speak and I mentioned it in my first post.) I haven’t poked around the Inns to find out about them though.