So I'm only doing one day, but, in general, it was a beast planning the two parks on two separate plans and then trying to work out the rest times on each to coincide with ride times on the other.
I did start with some of the pre made plans on the site and then deleted, added and moved things around.
As an HP fan, it was important to me to do diagon alley first (rather than hogsmeade). It was also important to me to do Harry Potter in one big chunk to get the full immersive experience (nerding out, I know) rather than bouncing in and out of the wizarding world. I started with UF for that reason and put in everything I wanted to do in UF. Optimized to see where things "should"go and then did minor rearrangements based on my preferences. This along with the existing TPs on the website, helped me determine where it made since to put my "rest" and hop to IOA.
once I had that figured out, I did my IOA plan based on my rest time from the UF plan.
As I got closer, I printed out the plans and compiled them all on one excel sheet so I could see it a step one big chunk and make sure the times all worked and made sense.
It all works on paper - will find out in a couple weeks if it works in practice too!