They were never a factory option. Various custom builders have done Moulton tandem conversions over the years (Doug Pinkerton springs to mind) usually working from an ATB or the more common (Pashley-built) APB model:
I think a separable/convertable solo/tandem is a great idea, not least because full-sized tandems are a real bitch to store and transport. A Moulton conversion is one way to do it. Another is an S&S coupled tandem, triplet, or quad from the likes of Co-Motion in the USA:
Would love a tandem for holidays with my better half but there's no way of squeezing one in the flat (else I'd be all over the Fisher for sale on here at the moment). But this... :shock:
Thats what I thought but the remaing front lower brace just above the cables doesnt seem to engafe abywher but I guess there must be a fitting on the front of seat tube?