I would suggest you talk directly to Thorn about this, but my guess is that they have already realised this and make the bikes so that a medium-medium would suit a medium man on the front and a medium woman on the back.
I'm 5'8", the wife 5'4". Our Thorn tandem is a Small-small, but could probably just about make a Medium-medium fit. We have succesfully ridden a large framed Dawes, although not for any distance (we hired it for a few hours at Rutland Water) and the seat pins were well down into the frame. I have no problems with having reasonable standover clearance on our frame, and I think it looks in proportion with the 26" wheels when there's no-one on it.
I would imagine that a medium-medium would suit you, but as I say, talk to Thorn. Obviously sizing isn't only based on height but also things like leg length. If all else fails, they say in the brochure that for a fee they'll do a special.