Actually got me thinking this; why does the women tend to be the stoker on a tandem?
Because men are control freaks.
The main answer is to do with strength - you typically want the stronger rider up front (I understand it's very different on racing tandems though) as they have to hold the bike and stoker up at junctions etc.
But it's not essential - there are teams out there with the larger rider at the rear (a team with a blind stoker for example cannot choose where that rider goes on the bike).
That's upper body strength, not cycling strength, btw. Chris & I are pretty much matched for speed on solos, as long as I've got gears- he's faster than me if we're both on fixed, but I have better stamina, and a stronger finish, usually.
I would struggle to pilot a tandem with my weight on the rear, although I have had my mum on mine and she's 5'8" to my 5'3", though probably not as heavy as me. It's not true that the pilot has to hold the stoker up at junctions or whatever- Chris doesn't hold me up (except by not getting going when I want to
), and I only hold up lightweight children, not the almost fully grown ones on mine. You do have to be a bit better coordinated with each other if you don't hold them up is all. I find as a pilot there's more 'steering' to do; on a solo, you just kind of lean, intuitively.
On a racing tandem you still need upper body strength on the front, you just can't get away with a passenger on the back. The gears tend to be on the back for racing which requires pretty good communication- we moved them on the Swallow as I didn't think it was wise for someone who can't see the road ahead to decide which gear to put it in.
The real advantage to the larger rider at the front is to stop the stoker seeing where they're going and demanding stops at petrol stations and the like.