TA makes craksets with inter-changeable spiders. Not sure if they make a tandem crankset but Peter White lists a 58mm granny diameter.

TACARSPI94x58B 94mm - 58mm Triple, Black, Smallest ring, 20 teeth

Talk to Peter (or maybe a British or French bike shop) and he should be able to tell youif such a beast exists.

I don't worry about the high gear much on a touring tandem. By the time you can go fast enough to use it, you might as well be coasting. I also don't like 11T cog because I can feel each tooth engaging. 12T is OK but I have yet to find 11S 12-42T Shimano cogset.