Is a 650B wheel good enough for a tandem?
The rim size is immaterial to the strength of the set you choose. 

Many rim brake 650b rims out there would not make good tandem rims, but there are exceptions. The VO Voyager or Diagonale in 36h are the main ones I can think of. Sun CR-18s are another, cheaper possibility. Also Velocity Cliffhanger or Atlas.

650b is likely the right choice for a conversion. If you happen to be maxing out your brake reach at present that could be a curveball, but at a radius difference of 1.5mm it's probably fine.