Very doable as long as your goal is to see selected attractions in each and not everything. 15th should be relatively quiet. We were there mid-December and did both. Got there at 9:30. Hit Forbidden Journey and Dragon Challenge twice each, Hulk, and Spiderman (skipped all the water rides unfortunately because it was freezing). We didn't do Seuss since we did that another day and didn't have kids this day. Went to Universal and did Rip Ride Rocket twice, Mummy twice, simpsons, and Men in Black. Left for day at about 5pm. So you can definitely do it. I think at that time of year, if you're skipping Potter it really doesn't matter which order - should be pretty low wait on everything else I would think.