Weight distribution is sort of hard to figure in, if you ride with various weight stokers/pilots.
You're sort of stuck with what you've got and captain has to be able to handle things.
Have ridden with sub-50-lbs to 250-lbs stokers. I'm rather light (135) but able to handle the situation. A bit tougher with the clydesdale stoker, but very do-able.
If weight distribution is a big issue, there are other designs than just 'upright' tandems.