As mentioned above, a stronger stroke with your right hand could potentially cause the issue. Concentrating on eliminating as much sweep element from your stroke as possible could help. And paying attention to the end of your strokes could help. The paddle should slice up out of the water cleanly as you’re finishing your stroke, making sure that the paddle never lingers momentarily in the water at the end of a stroke.
As a side note, I have no idea of the sizes of you or your partner. But boats do not behave well when they are bow heavy. If you’re the lighter person, you should always be in the front, and the heavier person in the back, in any tandem paddling situation. Either that or organize your gear so that the overall boat trim isn’t bow heavy, even if the bow paddler is a little heavier.
I have tandem kayaks and a tandem canoe. Directional control from the bow is difficult in any. It’s harder to detect the slight beginnings of a change in direction from the bow, and boats seem more receptive to subtle corrections from the stern. Bow turning strokes can be very effective, and are typically employed when the maneuvering is much more deliberate than slight directional control adjustments. The stern paddler should handle directional control, such as managing a boats tendency to turn slightly to the left. As I just mentioned, the biggest thing I would ever look for is the bow paddler having their paddle dragging through the water a bit versus a quick and clean exit after their stroke. But I have paddled with completely inexperienced people in the bow, as well as people where I found they could not manage directional control from the stern once we got paddling, and have always been able to easily manage directional control from the stern. Even with the kayak only slightly veering to left while you are paddling, I would still consider the stern paddler in the scenario. One possibility is that sweep strokes are more effective the slower you are moving. So the stern paddler may be able to compensate for a slight tendency to veer to the left when that person is the only one paddling, but add in a bow paddler, increase speed, the sweeps become less effective at turning the boat, so directional control has become more difficult for the stern paddler using those sweep strokes. If the bow paddler is the only one paddling, it’s much more difficult to control the boats tendency to veer slightly to the left, and it doesn’t rule out the source of the issue coming from the stern of the boat.
It seems I’m stirring up all kinds of mud into the water. I’d start with never having your paddle enter the water creating drag, or creating drag before leaving the water, as these are the most powerful sources of directional change in a moving boat. Then look at applying power evenly on both sides while eliminating sweep action in your strokes. Then look at the stern paddler’s ability to manage subtle directional control at speed. “Take us over to the right.” “Take us over to the left.” Just see where that ability is at. The other good result of this practice is that you will find that the stern paddler can do the most effective maneuvering work if the boat maintains momentum through the bow paddler never stopping with their normal forward stroke. You say “Take us over to the right?” and you keep paddling forward, paddling to keep momentum, with no changes in your storke to help effect the turning. Is the stern paddler getting the entire paddle - both blades - on one side of the boat and initiating a nice, efficient stern rudder? Is the stern paddler simply pushing one blade of the paddle down on the water beside them, doing more braking than turning?
Any tandem paddling pair can learn a lot through attention to detail during exercises like these. You say right now it’s bothering you because your partner has to correct it while paddling. Any directional control effort the stern paddler does takes something from forward effort, and so I can say that while the stern paddler is providing directional control, they are feeling bad that the bow paddler is cranking out all the forward power. So becoming efficient in directional control from the stern should be a matter of respect towards the bow paddler as well.
We love tandem paddling. It truly takes on a team feel. There are rewards that simply aren’t there while solo paddling. Good luck and enjoy!