The way they switch is actually interesting. Superchargers are made from (I believe) 12 smaller chargers that each max out at 12 kW. 3 are always grouped together as they use the three phases from the commercial grid power. When both stalls (that share a charger) are used, the power output can only be switched in groups of 3. So when you arrive and the other car is using all the power, you will get the minimum which is 36 kW. Once the other car tapers down enough you will get another group of three which adds up to 72 kW. The next step is 108 kW.

Here is a graph shows the charge rate at a Supercharger. It started with 36 kW, the switched to 72 kW and after 6 minutes it went up to 108 kW. At that point my battery pack wasn't able to take as high of a rate so the power dropped down gradually as the battery fills up.

BTW, urban Superchargers don't have the issue. They are limited to 72 kW but every stall has it's own dedicated charger.