I have done multiple HIIT workouts in a single workout session if I don't feel that one was enough, but then stay away from HIIT for 2-3 days as Pcramer suggests.
I've heard the sessions are short so you can give it 100% and go all out. If you did two back to back, the second half would probably be more of a regular interval workout because you wouldn't be able to sustain the very high intensity from the first half. Not sure if it would be detrimental..? I'm not motivated to want to try it.