That is a pretty disturbing photo and has always been my single biggest fear when riding our Tandem, even more so than getting hit by a car which logically I know is certainly a bigger likelihood since some of us have been there on a single.
We also have a steel 1'' on our C'dale that we ride a lot on hilly rough roads... Since moving to the mountains years ago, I religiously pull the fork and inspect it the best that I can several times a year. I usually focus the inspection on where stress riser's might start, specifically at the end of the tooling marks on the top threads and at the 90 degree angle where the head tube meets the crown and the lower bearing race sets....to see your's fracture in mid tube is particulaly scary because that would have been the last place I would have expected a breack to occur.
Knowing how hard and fast you guys ride, I'm really glad that very bad things didn't result. Time for your Calfee for sure!

Bill J.