If Potter is your focus you need a park to park ticket to ride the Hogwarts Express. It's well worth it. We never get tired of it. It is part of the full Harry Potter experience. Not riding the Hogwarts Express would be like going to
MNSSHP and not trick or treating.
Go to the Universal website and look at the park maps. This will give you the lay of the land on both parks. I will help a bit with the Potter things.
1. Get the girls interactive wands to do the magic tricks at Hogsmeade and Diagon Alley. The map for the magic sites are included with the wands. TM's are standing at magic site areas sometimes to help. There are also metal medallions on ground in front of magic sites marking the spots.
2. Have a butterbeer at the Hogshead Pub in Hogsmeade.

Have lunch at Three broomsticks.
I like the Three Broomsticks food better than the Leaky Cauldron, but they are both a must visit for the Potter fan.
3. See the Ollivanders wand show in Diagon Alley as it is less crowded.
4. Don't miss the Parcel Tongue snake in the side window of the Magical Menagerie store in
Diagon Alley.
5. Listen for the moving bricks as you enter Diagon Alley.
6. Go have a visit with the Bus conductor outside Diagon Alley and listen to the shrunken head
Talk to you.
7. Have a ice cream at Florean Fortescue's.
8. Try on a robe at Madam Malkin's robes and stand in front of the talking mirror.
9. Get pic of fire breathing dragon on top of Gringotts Bank. Occurs every 10 a 15 minutes.
10. Visit Borgin and Burkes in Knockturn Alley.
11. Toothesome is worth the visit
12 Mytho's in IOA has very good food.