To the best of my knowledge, there is no such animal. Tandem producers switched over to 1.125" and 1.25" steerers in the early 90's shortly after the necessary headsets were standardized by Tange and DiaCompe. Therefore, the "demand" to develop a 1" super-robust fork beyond cyclocross applications is pretty small.
That's not to say there haven't been others in your situation who have opted to accept the risks associated with using some of the more robust 1" CX forks on tandems without the manufacturer's endorsement. However, IMHO unless you had access to the static and fatigue test data for that fork and had the knowledge to analyze it against the loads it would have to manage on your tandem, you'd be headed down a high-risk path where the potential benefits of using the fork could never be justified.
I should note, you'll definitely find a few tandems with very lightweight teams riding around on 1.125" carbon forks that were not designed for tandems. In a few rare instances, the tandems were actually sold that way after an "off the record" discussion between the tandem builder and the fork's manufacturer that purportedly supported the notion the non-tandem rated fork "probably" had sufficient strength for use on tandems with a total weight of not more than XXX. To be clear, the fork's manufacturer did NOT endorse the use of their non-tandem rated forks on tandems and clearly indicated that using one on a tandem voided the warranty. I have not heard of any problems from the folks who purchased these tandems; however, the tandem brand in question ceased to be a going concern in late 2003.
Wilier Zero 7; Merlin Extralight; Co-Motion Robusta; Schwinn Paramount; Motobecane Phantom Cross; Cervelo P2; Motebecane Ti Fly 29er
I wouldn't want to descend on a tandem with a fork with a 1" carbon steerer tube. However, there's no inherent reason you couldn't make a carbon fork, with a 1" steel or aluminum steerer tube, but there's no market for it.