I managed to get the crank off by using one of those three-arm extractors (which grip the crank from behind with large hooks), and a huge washer, in which I cut a slot to fit over the BB axle. Then the extractor arms could grip on the washer. Without washer, they slipped off.
Others recommended converting a crank extractor into a thread cutter by filing a few slots, and tapering the threads at the end slightly. I may have to do that to restore the threads at some point. Thanks for all who responded.
BTW, the tandem is a 1940s Herse, and has tapered roller bearing BBs, made by/for Herse, with special spindles (LH thread on the left side crank bolts), pressed-in races, and more weird stuff. Hopefully, I'll show the various Herse BBs in a future issue of Vintage Bicycle Quarterly. Sometime before 1948, Herse went to a simpler arrangement, which I have not seen apart...
Jan Heine, Seattle