Front or rear standover height?
I'm a fan of traditional sizing, or Rivendell-like sizing, which means "get as large a bike as you can straddle". My tourer (single) is a Trek 520 with a 25" frame, and I could have used a 26"-26.5" frame. Yet my tandem is a Co-Motion with 23"/18" frame... and I might have gone as high as 25", no more.
Why? You need to have enough seatpost exposed to attach the stoker bars. That means about 80 mm of seatpost, just to attach the rear stem. Also, the beefier structure of the tandem makes the larger seatpost more sturdy than on a single.
Also, as you can't swing your leg through your stoker's face, a slightly lower top tube helps one to straddle the bike.
Frame size is also affected by standover height, and I'm told that tandems typically have a higher bottom bracket than singles. However, my tourer and tandem come within a few millimetres of eachother.
For the stoker standover height, I'd agree with what was said. Stoker needs to be able to pedal, but he/she won't step down until destination.