Well have been to HHN a few times now, and I have to say get the fast pass if you are serious about doing the haunted attractions. Plus if you didn't know, there is an entrance for HHN in island of adventure. Its usually right next to one fish two fish ride. They should have a sign for it ( thats the way it was last year ) But yeah that way when Universal closes for HHN set up you can head over to IoA and hang out for a little bit and then just cross over when its open. But I guess that only if you have tickets for both parks, but I sure tell you, entering this way will save you a ton of time instead of piling in with all the people at the front gate I guess thats the only tips I can give lol Hope this helps in some way
p.s. if you want to use the IoA path over to HHN you have to have pre purchased your tickets FYI