thru axle stiffness I thought was more applicable to suspension forks where the two legs benefit from a stronger tie together as they shock. For a rigid fork I don't think the thru axle matters.

FYI, Niner makes a wicked carbon fork for disc, with NO RIDER WEIGHT LIMIT. More than 100g heftier than the enve, so I tend to believe in the strength. And as a rigid mountain bike fork it should be expected to hold up. But the 470mm axle to crown height far exceeds the 367mm for a road tandem, so it will not fit here. And the niner has a tapered steerer tube, so would need the appropriate straight 44mm frame head tube.

Ritchey makes a carbon disc fork, no weight limit immediately apparent. But again the 459mm axle to crown dimension does not work for road tandem.