The twice a day park concept would also allow the opportunity to increase overall turnstyle.clicks. If the park could sell out twice.a.day, the theoretical capacity would be 4x of design.When you add an attraction, you attract more people to your park, not less. Especially a new one.
Take for example, TRON. At any point during the day, there may be 500 people in its queue on on the ride. Then that is only 500 people less milling around the park and in the restaurants on other attractions. With nearly 30 attractions and 20 'other' venues (restaurants and stores), then TRON would mean there is only 10 people less at all the other venues. Unless... the building of TRON brings in more than 500 people per day, and then the building of TRON would make the park more crowded, not less.
Redesigning rides to increase capacity is a huge endeavor and usually requires using up more space (like with TSM and Soarin).
If anything, selling the park twice in one day would cut crowds. A morning ticket of 6a - 4p; and an evening ticket of 2p - Midnight would give each group 10 hours and nearly cut crowding in half.