A stand-up special is usually recorded at two shows on the same night rather than at one. The reason is that comedy allows almost no retakes, so the second performance is the only safety net available.
There is no way to reshoot a joke
A scripted scene can be repeated until it works, because the audience is not present and nothing is lost by trying again.
Stand-up depends on a room hearing material for the first time, and a joke repeated for the cameras receives a different and weaker response.
Recording two separate shows, each with its own audience, is therefore the only way to obtain two genuine performances of the same set.
The two shows differ more than expected
Rooms have their own character, and the same material can land very differently depending on how the crowd arrived and how the opening went.
Comedians also adjust between performances, tightening a phrase, cutting a preamble or reordering bits based on what worked an hour earlier.
The later show is often sharper as a result, which is why editors frequently build from it and use the earlier one for repair.
The edit assembles a performance that never happened
Editors select the strongest version of each section and join them into a single continuous set.
Because the comedian wears the same clothes and the stage is unchanged, the joins are invisible provided the audio is handled carefully.
The finished special is therefore a composite, presenting as one uninterrupted evening something assembled from two.
Laughter is the hardest thing to join
Audience response is continuous, so cutting between shows risks an abrupt change in the level and character of the laughter.
Sound editors smooth these transitions using room recordings, matching the tail of one laugh into the start of the next section.
Getting this wrong is immediately noticeable, which is why the audio work on a special takes longer than the picture edit.
The material was finished long before
By the time a special is filmed, the set has usually been performed for a year or more in clubs and theatres.
Every line has been tested, cut and rebuilt in front of audiences who paid to see work in progress, and the filmed version is the settled result.
The two-show recording is the last stage of that process rather than the start of it, capturing an hour that has already been optimised as far as it will go.