HHN at Universal is great and it is very, very different than Disney's "Not-so-Scary Halloween Party" as they try to make it very scary. We'll be there in a few weeks (I have 8 days staying at the Universal Portofino Bay hotel) and have reserved a Universal RIP tour for the family to hit all the scare houses in a single night (and we will go a 2nd night to HHN to re-hit my daughter's favorite scare houses and scare zones). The problem you are going to have is stamina. As The _Banking_Scot mentions, it will be a very exhausting 2 days. What are the 2 days you want to go? If they are a crowded days, then there will be no way you can hit everything both of Universal's parks in a single day without having an EP. And even then you probably won't be able to hit the newest Universal ride, Hagrid's Magical Creatures roller coaster ride as there is no EP for it yet and the lines are anywhere from 1 hr to 3 hrs.

Note over on the Disboards, I remember a post that someone said they tried to hit all the scare houses in HHN in a night (using a separately purchased EP ticket which isn't the same as the Park EPs during the day), they counted over 16000 steps that night which is probably around 7.5 or more miles of walking. So if you are hitting all the parks during the day, you can bet you will be more than 10-15 miles of walking, plus another 5-7.5 miles of walking that night, plus another 5-10 miles of walking the 2nd day (if you are trying to max out both Universal parks, HHN and Epcot in 2 days.) That's 30 or more miles of walking in 2 days. It will be an exhausting 2 days and not what I would think of as "fun". But everyone is different and can define "fun" however they like.

My suggestion is pick either Universal or Disney and buy a 2-day pass. It will be less expensive "per day", save you time from traveling between the 2 and let you focus on maximizing your experience at either Disney or Universal. For example, if you were to pick Universal, you could stay the night at one of Universal's 3 Premier hotels and you will get 2 days of Unlimited EPs. With 2 days with EPs to walk through both Universal's parks, you should be able to hit every ride you want (and even reride the ones you like), all the while not killing yourself trying to hit everything. You won't have to hit the parks at rope-drop in the morning, stay till closing, stay at HHN till well after midnight and then get up to do it again the next day at Epcot.