It really depends on what you want to get out of the party. If you are looking to include rare characters meet and greets, then you may be better off with two. However. if all you want is short ride lines, the fireworks, and the parade, then once may be enough. The first year we did it, we did the trick or treat and decided that was enough of that. Collecting candy that, for the most part went uneaten, was a nightmare and really took away from the experience. I can't suggest strongly enough to avoid the trick or treating.
The most important thing, though, is to make your party day(s) non-park days outside of the party. In other words, do not use a regular park ticket to get there at noon and expect to be able to fully enjoy the party, It just won't happen. Arriving somewhere around 5:00-5:30 allows you to get the most out of the party without being totally exhausted by midnight. And, the later you stay, the better it is. Last time we went, right before midnight, we were able to stay on Big Thunder Mountain and Splash for a second ride, not get off and get right back in line, but actually stay on the rides.
Finally, nothing ruins a party worse than an uncomfortable costume. It is easy to get overheated and once that happens, impossible to cool back down. Make sure the costumes have shoes that are easy to walk in, fit easily on all rides you want to do, and are well ventilated.