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Ah, you're probably right. The braze ons that litter the frame seem to suggest that I should be able to run two rear brakes.
In the top-right of this photo you can just about see the double braze ons up near the pilot's head-tube. The secondary cable would then run through the braze-on next to the pilot's seat-tube, then through the two braze-ons under the stoker's water-bottle mounts.
But how do you run two rear rim brakes from one brake lever? And is it unusual that the seat-stay brake braze-ons are 75mm between, while the "middle-stay" brake braze-ons are 65mm between? Certainly the one U-brake in my parts box doesn't fit there.
I can see cantilever brake bosses on the seat stays but are those additional ones on the "middle" stays for a U brake?