This question is related to this, and perhaps others on the site, although I don't think it's an exact duplicate.
I have friends who would like to take a two-person kayak trip down a stream in the White Mountains region of New Hampshire, USA, while on a vacation. The boat would be a rental from one of those places that caters to vacationers.
One friend was born without arms, and therefore has no ability to hold a paddle. They're trying to figure out if they should have her sit in the front or the back.
My fully-abled friend has done the same trip a few times, so she knows the terrain, but she's only ever used a single person kayak. The stream in that area is fairly level, with some curves, hills and rocks, but nothing too strenuous. It's not deep enough for capsizing to be a concern.
My handicapped friend misses out on a lot of fun on vacations, because she has to wait on the sidelines while the rest of our friends go on excursions without her. That's why they've decided to rent a tandem kayak this time.
It's possible that when they get there the rental place will suggest who should sit where, however, they'd like some advice before they go.
Both women are about the same height and weight, so those will not be factors. The only difference will be that one person can't hold a paddle, or anything else.
A canoe is not an option, as the place they're going is too narrow, and canoe rentals are not even available for that route. They could choose a different area and use a canoe, but they like this route as it offers the most options for seeing birds and other wildlife.