The way Tandems are made- unless you get a custom made frame- The rear compartment has a shorter cockpit than the front- hence the shorter rider is on the back. I have seen Females in the pilot position but that is exceptional. On top of that it does take a lot of upper body strength to control a Tandem so it is normally the Bigger- heavier rider that will be in charge of steering.
As to females always being towards the rear in solo group riding- I would put it down to politeness. The Macho man will always try to shield the female rider from the wind and traffic-----------Providing he can catch and overtake her.
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How long was I in the army? Five foot seven.
Spike Milligan