You should probably contact them directly. I went there last year for a day trip and it was wonderful. They are very helpful, and I believe they offer packages. Their web site posts their schedules, so maybe you should plan the date around that.

Here is a link that might be useful: Longwood Gardens PA website

I see you posted this a while ago but if you are still asking I'm posting some info from my trip last June. We visited Winterthur the afternoon we left home. The good thing about that was being able to come back the next morning with the same ticket. We didn't come back in the morning but had a much more enjoyable and relaxing time wandering around
Winterthur knowning that we'd be able to be back the next day. We walked everywhere but there is a little trolley-type bus that can pick up and drop off visitors in different parts of the estate/garden. We spent the entire 2nd day at Longwood (in the gardens, woods, conservatory, fireworks show at night) and it was incredible. We spent the third morning at Nemours, took the house tour (you must make reservations in advance) which was very interesting and then had time to walk in the gardens. Most people spend a short period of time in the gardens and go back with their bus but you are allowed to spend all day in the gardens if you'd like and return with a later bus. It was misty/rainy and they gave us umbrellas (in addition to juice and cookies at the end of the very small group tour). We spent a little extra time but the weather wasn't holding up and the bugs were out. But each garden was very different and very much worth a trip of its own. You will wind up unexpectedly familiar with Du Pont family history after the trip. We stayed at a bed and breakfast near Longwood but I can't find any of the information about it. Some of the conservatories are closed at Longwood but it is so huge that any construction shouldn't interfere at all.