The big issue with two sets of races is lost/down time between races.
Timing and Scoring, track clean up and grid need time, in most cases 3-5 minutes.
With 8 run groups that's about 30-35 minutes lost, now add lunch and a shorter day.
How long can the races be? 12+/- laps or 15 minutes?
Would racers be happy/satisfied?
What kind of effect does Friday qualifing have on guys who can't get the day off?
Will they still come? Would the first race be the qualifier for the second?