If you work first heat and drive second, you can leave right after you finish your last run. You could be out around 2-3ish in that case.
If you run first heat and work second, you have to work until the end of the second heat.
If you want to run second heat so you can leave early, I recommend getting there EARLY to help set up. It gives you a better chance at running what heat you want as EVERYONE wants to run second heat. The regulars remember those who help out more when assigning work/run assignments......