If you buy your tickets online from USH you get access to Harry Potter an hour before the park officially opens. My DH and I were there on a Wednesday in early November 2017 and were pretty much done by lunch time (Note: we didn't do the shows or the studio ride). We didnt get the front of line pass, but as noted above you can buy it on the day if you think you need it.
We rope dropped with the early access* and rode all the HP rides with no more than a 5 minute wait, had a wander in Hogsmead, did the wand choosing and were out in the upper lot well before 10.00 am the official park open (rides were open in the upper lot by then). The lower lot was closed until about 11 am and we actually had time to kill (in the shops ;-) ) before they'd let us down there.
* They held us at the rope drop near the customer service centre close to HP until 9.00 am, and because it was a small group waiting the guide let people through to the service centre to buy FOL passes while we were waiting. Personally I dont think they needed them at all that day.
My advice is get the early access ticket, and as it's just the two of you you'll probably be fine and be done by early to mid afternoon even if it's a bit busy. Have a great time, the Revenge of the Mummy ride was awesome and has a single rider line if you want to use that for multiple rides.
PS: if you get motion sickness remember to take your tablets well before you get to Harry Potter, Forbidden Journey will get you otherwise!