I'm curious how you get around normally being able to only run one instance of the iOS simulator at a time? Having a load of OS X machines for each individual s

I'm curious how you get around normally being able to only run one instance of the iOS simulator at a time? Having a load of OS X machines for each individual stream or even virtualised hosts cannot be the case surely? Either way it's done, it's a very impressive way to show off your apps and works suprisingly well.