Lots of people do this, on the PCT and other trails. My own wife is a bit too sane to want to hike continuously, but she's trained to hike hard and joined me on stretches where --- surprisingly perhaps (not the typical experience) she's come very quickly up to speed and fit right in.
I think the big issue for couples of any sort (a pair of friends or a committed couple) is to decide what to do if one of the pair wants or needs to leave the trail. Will both leave then, or will just one?
I recommend that whichever you *think* you'll want to do that you make gear selections to be as independent as possible. Think of yourselves as two independent hikers who choose to hike together, rather than as being joined at the hip. Even if both of you stay together continually, you might find that you want to walk apart for significant chunks of the day; my wife and I do that, and I feel better knowing that she is (and I am) "self-contained".
On the PCT I was acquainted with a nice couple in their 50's (I would guess, my age range too). A few hundred miles in she had had enough and opted to go home. He continued on --- with a 2-person tent, large cook pot, that sort of thing.
OTOH, on the AT a very young couple was looking to replace their really old and sketchy tent. After looking over my tarptent contrail --- a fairly spacious solo tent --- they bought one of those, rather than going for a bigger, heavier two-person tent. And they seemed happy in that, though of course on the AT you can sleep in shelters a lot; still, I saw them in that tent several times thereafter.
Bottom line though, think at least tentatively about what you'll do if one leaves the trail and talk that out.
As far as one partner needing convincing to do this --- I think that the best convincing would be to go out together on a long-ish hike, at least 50 miles, talk about it on the hike. There are so many factors, just in terms of where you are in life --- jobs, finances, pets, family, friends, other opportunities, that even if you both would be happy living like hobos for months, it might nevertheless cause too much stress. I was fortunate that my wife was clearly against doing it, so there wasn't any temptation to try to push her into something she didn't want to. The successful couples that I've met (and a good few of those) *both* of them seemed to both do very well on the trail, seemed to thrive on the experience.