Yes, it will be very crowded and very hot!! Be prepared for long lines for everything.
I think doing two parks would be extremely hard. I would stick with DL. My advice:
~Fast passes are your friend! I would hit the big three (Space, Indy, and Splash) first thing after the Subs. Get fast passes for those three and go on them while the line is short. I'm not sure on holidays, but DL doesn't get crowded till about 10 am.
~Save your fast passes for later in the day when the lines are really long. The CMs will let you use the fast passes after the time frame.
~The last two hours before the park closes are great! After the fireworks, many people leave. It's not crowded, cooler, and shorter lines (most of which are a walk on).
If you want to go on the Subs, you must get there before the park opens and run straight there. I went a couple weeks ago and got there 10 minutes after the park opened to find a 3 hour wait (and growing). The line will most likely reach IASW on Wed.