After spending weeks troubleshooting similar noises on our tandem and replacing both bottom brackets, etc., I would just caution that it is very easy to be fooled by the source of noises on a tandem. In our case, it turned out that the noise originated in the rear wheel, even though it happened once per crank turn. It's helpful to use a stick or tube as a stethoscope to home in on the source if you can replicate the noise when not actually riding. On tandems, it's common for the stoker's BB to go first as it carries twice the power, so be careful about focusing too quickly on the captains BB.