Hi Debra,
I have been hearing more and more guests talking about trying to make it to both
Mickey's Not-So Scary Halloween Party AND
Mickey's Very Merry Christmas Party on the same trip, I think its a FABULOUS idea! Not only will you experience Walt Disney World with all of the beautiful fall decorations, but you will witness the transformation from fall to the Winter Wonderland! There is a possibility for you to make this happen, it will take some careful planning, but it is certainly manageable.
To make this happen, I would recommend booking your vacation where you arrive on the LAST day of the Mickey's Not-So-Scary Halloween Party, and attend the party on that night. This way you will not use a Magic Your Way Base Ticket for your arrival day. It seems like there is around a week turnover, so you should be able to experience Mickey's Very Merry Christmas Party toward the end of your vacation. To make sure your dates coincide, I would recommend double checking with the schedule for both of the special events, before booking your vacation. The official dates have not been announced as of yet, so keep checking the Disney Parks Blog or the Walt Disney World Website as information will be released here first.
Wishing you some extra Pixie Dust to ensure your planning is s smooth as possible!
Stephanie