You should be able to, but you won't be able to start downloading your first set of pictures until just before you leave for your second trip and you will have to have your downloads complete just a few days after you return from your second trip.
MM entitles you to 30 days of downloads. after you download your first picture, you have 30 days to take more pictures, make edits, and finish your downloads. Photos are good in your account for 45 days from the day they were taken. So the pictures from your first trip will expire just before Christmas. If you wait to start downloading until Christmas week (around the 21), you would still have until the third week of January (MLk weekend, one week after your trip) to finish making edits to your second set of pictures and complete your downloads. The first set of pictures will expire before your trip, so make sure you have any that you want downloaded prior to the second trip so you don't lose them.
*****Make sure you check your dates prior to doing this- I know it will work because we had. 2 trips over the span of 7 weeks during the summer, but I don't want you to possibly lose your pics if my calculations were off by a few days.