If we are a party of 6 and half want to go to HS and half want to go to AK. We can get fast passes for both parks on the same day, right? 3 of us get HS and 3 of us get AK.
What you need to remember though is that if plans change, you can't swap people around and be able to use their fastpasses. You would need to try and change them.
Yes, it asks you to "select fast pass party" if you control all of them. Just select each group.