Reddit mentions of The Barefoot Sisters Walking Home (Adventures on the Appalachian Trail)

The Book Appalachian Trials by Zach Davis is about the AT, but it has some good information about the pros and cons of hiking with a partner, as well as what is necessary to make it work, that are applicable to any long trail. It covers the topic better than any other "how to" book of long distance hiking I've looked at does.

Another set of books that is also AT specific but still probably relevant are the Barefoot Sister's duology of books about yo-yoing the AT together- Southbound and Walking Home. They definitely had some blowups/fights along the way (some of which ended in tears), and the books are surprisingly honest in their descriptions of this psychologically difficult aspect of thru-hiking.

There is no guarantee that you guys are going to be compatible thru-hiking partners. And even if you are, it is still going to take willingness and effort from both of you to make it happen successfully. And even then, you're probably still going to have a couple of (potentially nasty) disagreements along the way. I agree with the others that working up to a thru-hike through some (relatively) shorter hikes that still require you to spend a lot of time with your SO is a good idea.

But don't just discount the idea because it is difficult or because you find major obstacles along the way to making it happen- being able to do something like this with your SO is a great way to become even closer with that person.