DEEDLE DEE DEE DEE, TWO UNITS
We have one first unit and one second unit shooting. The second unit is smaller and in general is not expected to shoot drama. There is often a second unit director although you can also try to get the episode director to oversee his second unit so the style all matches.
We also have two sets of ADs. One set preps the next show while the other set oversees the shoot of the current show.
This means that there's a limit to how much advanced prep can be done. The HODs (heads of departments) need to know what items they'll be responsible for (set dressings, props, locations) but the ADs aren't really available until their current show is done shooting.
So while we've been working to get the shows written two weeks in advance of photography, they don't really HAVE to be done until one week in advance of photography. But of course at that point they must be as perfect as they can be. Once prep has begun on an episode, changes become progressively more expensive.