The Bus..

Well the other side is this.

If they'd had half the surgeons at the conference, what happens if they get an influx of wounded? Shift doctors from another unit to the 4077th? Then if two surgeons are at the 4077th from the 8063 due to an influx, what happens if there's an issue at the 8063?

Even great surgeons from a different unit might not work as well with the remaining doctors:

They wouldn't know who's better at head wounds, chest wounds, and such. Even a delay of minutes could mean the difference between life and death if you don't know who's a specialist is for serious injuries. Secondly, the nurses would work far better with doctors they know.

The doctors from the 4077th, nurses, and corpsmen work as a team. It make things in Pre-op, OR and Post-up work far far smoother. A visiting doctor wouldn't know that Klinger, for example, was one of the most dependable corpsmen, and Rizzo was probably the worst. Seeing Klinger in a dress they might wrongly assume he's a loafer, and that Rizzo, in uniform, was a model corpsmen. Since minutes, if not seconds, matter, if Klinger were brushed aside for wearing dresses, and a visiting doctor relied on Rizzo, it could lead to problems.

Also how would the two doctors take needed breaks if there's even a light influx of injured.

Consequently. being that the units are not that far apart. Like we find in "Heal Theyself" the 8063 is only minutes away by chopper, they likely just don't take wounded to the 4077th for the few days for said conference. They could have the conference run for a week straight, with the 4077th there Monday, 8063, Tuesday, etc..

Destroying an empire to win a war is no victory and ending a battle to save an empire is no defeat

They weren't the only surgeons in Korea. I'd guess that some replacements were at the 4077 for a couple of days.

A real MASH would have had 10 or more surgeons so having 4 attend a conference would not have been much of a problem.