I ended up upgrading to DaVinci custom cranks. They cost more than the complete tandem did. They gave options for 150 or 160 so I opted to go with a ratio of crank length to inseam length.

170mm x mm
------ = -----
31.5" 27.5"

Solving for x gives 148.4 mm so I went with 150mm cranks. I'll post more when I get them in hand and installed.