I wouldn't worry about it. Assuming the headset is properly adjusted and functioning well. And there's enough overlap of the quill within the steerer tube. (If in doubt, drop it below the max height line a few cm for peace of mind.)
Think of the difference in leverages on the steerer tube:
1) Your upper body on the handlebars. Sure, you're exerting more leverage, but it's just you pushing with your arms. Not even your full body weight. Unless you're planning on doing handstands on the handlebars. And during a hard stop, you are exerting more force to the bars, but still not more than your body weight.
2) The fork legs. The fork is supporting both riders riding through potholes, bumps, rocks, roots, you name it. That's a ton of instantaneous force acting on the fork blades, which are even longer than your quill/stem/handlebar lever arm. And then think of panic braking. That force is acting perpendicular to the head tube (mostly) and utilizing the entire fork as a lever arm. And it's having to withstand the braking forces exerted by two riders. Now THAT is some MAJOR leverage on your fork's steerer tube. So if this is indeed a tandem fork, then it's been designed with these incredible leverage forces in mind.
Now the only concern I might have would be the forces exerted on the head tube. During a panic braking situation, you would be loading the head tube considerably, and from both sides. During hard braking, your front wheel axle is pushing back hard on the fork legs. And you're pushing forward on the handlebars - and even moreso with a longer quill. So you have a sum of these two forces acting on the head tube pulling it forward off the top tube, and backward against the downtube. THIS could possibly be problematic. So if you DID have a failure, I'd expect it to be along these lines.
But if this is a tandem frame worth more than junk,* it should have been designed to withstand these forces.
*After a closer look at your picture, I see it's a Burley frame. This is a respected tandem maker, so you should be just fine.
Last edited by LV2TNDM; 07-14-22 at 04:35 PM.