I do not think you can do both on one night. We actually booked two evenings at HS for this purpose alone last year. And JBJB is fun, but if you have not seen Fantasmic and you are picking one, Hands Down choose Fantasmic, HOliday or no Holiday.
Fantasmic is just awesome - everytime. The first time, it will blow your mind!
The crowds at night during holidays is NUTS! We were there Dec. 14-20 I think last year - not even Christmas day and the crowds in no way would allow even moving between those two spots quickly, much less getting a seat. I would get to Fantasmic's 8:30 show at 8, or you may not get a seat. It is a VERY long que, and a huge stadium. You cannot show up at 8:25 and get a seat during holidays, has been my experience.