I was wondering the same thing! My sister and I will be in UO/IOA on March 13th (which is also a Mardi Gras night). We unfortunately only have one day to do both parks, so have EE, as well as park-to-park tickets w/ unlimited express. We've been before, but that was before any of the Harry Potter or Simpsons was even built! We are guessing that EE will be at IOA and were thinking it might be best to just take Hogwarts Express immediately to Diagon Alley to ride Gringotts and enjoy DA before it gets too crowded. Would you recommend fitting in a ride of FJ, assuming we get in at rope drop before heading over to DA? I don't want to waste 30m of our time waiting for Hogwarts Express to begin if we could probably catch the first train after riding FJ.
Also, I was planning on just spending most of the morning in UO because of Mardi Gras. I thought I read somewhere that they close down UO for the parade/concert, so I figured we'd just spend that time in IOA (hopefully with less crowds since they'll be at Mardi Gras!). Does anyone with more experience with the parks and Mardi Gras have any recommendations? Is there a better plan? I'd really appreciate any help the DIS experts could provide!