2-day definitely if you have not been. These are both large full-day parks for first timers. Park-to-park option is important to get the full Harry Potter Experience - specifically the Hogwarts Express that runs between the two parks.
FOTL access is nice if you stay at resort that includes it, but it is also available a-la-carte. We have never purchased FOTL and with proper touring planning isn't generally necessary. Note that FOTL pricing is demand controlled, and gets fairly expensive at busy times (we are March breakers, so it's expensive then), but is probably completely unnecessary when the price drops.
Finally, be prepared for you 12yo to possibly enjoy this park more than Disney. IMO Universal has been out-Disneying Disney for a while when it comes to theming and attractions. I know my kids both prefer Universal to Disney when given the option. You may be opening a Pandora's box here. We now send our older teens off on their own to USF/IOA while DW and I go to MK or Epcot.