Simulator session times out when switching between platforms

[kgb]

We're using Zapp to run our KIF test suite on iOS 4.3 and iOS 5.0. If I queue the same build twice in a row, our tests run and pass or fail as appropriate. However, if I queue one build and then the other, the second always fails before running the tests, with this error message:

Error Domain=DTiPhoneSimulatorErrorDomain Code=2 "Simulator session timed out."

Since both builds are queued when a new commit is pushed, we're always switching between platforms, so our tests always fail to run.