I got the following private message, and think the questions are important enough to post publicly:
I have done whitney 2 times, and both the times I have done it in a single day. However, this time I will be taking less experience friends and plan on doing it overnight. I was able to get an overnight permit on Friday Aug 9th. I am not exactly sure how many days you are allowed to spend on the mountain with 1 overnight permit.
Also, do you have a recommendation on how people actually do the overnight? Hike up, set up tent and leave gear and continue to the summit, then come back to the camp (below switchbacks) and spend the night and then go back down the 2nd day? I would appreciate any suggestions, or resources so I plan this trip right.
The overnight permit allows the permit-holder to stay as many nights as they want! The only restriction is that after 2 weeks, you MUST move to a different camp site -- the no-squatting rule.
The one important point on the overnight permit is that you MUST start your hike and spend the first night on the date specified on your permit.
As for doing the overnight hike: Only the strongest carry their packs up and then summit the same day. Most are pretty well wiped out carrying (often too much) gear to their camp site. And due to the weight of that gear, I would recommend camping overnight at Outpost Camp, rather than Trail Camp. Second reason is that, unless people are well-acclimated by staying overnight at perhaps Horseshoe Meadows the night before starting out, the Outpost elevation is easier on the body. Some people pack all the way to Trail Camp, and then due to the elevation and exhaustion, cannot go next day to the summit.
So by staying at Outpost, you get up bright and early, and hike to the summit. Just try to find a spot to camp overnight well away from the trail, since there are a few inconsiderate day-hikers who come through super early and wake people up with their loud voices.
Anyone/everyone: please post more questions and suggestions that might be helpful.