If your concern is getting in everything unique to the Christmas Party, you can definitely do it in one night. It's a great event and I wouldn't miss it, but there aren't but a handful of things different from a "normal" December night at MK (parade, a show or two, cookies, hot cocoa is all that comes to mind).
Now, if money were no object and I had the opportunity to go twice, I would not hesitate. But, for what they charge, once per holiday trip will have to do us.