I'd prefer 2 shorter races.
My reasoning; I'm trying to learn as much as I can about racing and I really only gain this from being in races (trackdays only offer a certain amount of learning opps for me) and then the reflection afterwards of where it went wrong/right.
The opportunity to have 2 seperate races on the same day would offer me the considered opportunity to "commit myself more to the start", "brake later", "try a different line" etc, etc...... this after taking the time to reflect on the first race's shortfalls.
As an aside, I'm learning that its a lot easier for me to go faster when I'm chasing someone than when I'm out on my own - this does not necessarily happen on trackdays nor even in qualifying.
My best laps are when I'm racing closely with others. I reckon that pretty much wherever you are placed after the first lap will dictate (give or take a few places) where you'll end up. You'll spend your time jostling with the riders around you and using blocking lines, etc, etc.... that you might not be able to easily break out of that group.
So back to the point;.......... more races = more time to reflect / regroup and hopefully improve. The idea of different formats is interesting, but I'd still go for 2 races a day (even of 30 mins each).
How about a Le Mans style start????
Tim