For those crank lengths you are looking for it may be tough. No doubt I'll post this and a set will immediately become available.

FWIW, you may be more cost effective to build a set. You have seen mention of re-threading and heli-coil inserting standard crank arms. As an even simpler solution, there have been some off-road tandem cranks converted from standard cranks, where just new threads were cut, or the pedal axles converted.

I am not sure of all your build details, you may be able to use an all right side drive utilizing the inner ring for timing chains.

Middleburn makes a worthy product. Many tandems run them off-road. Middleburn stuff is available through Alex at MTBTandems.com. They offer the sizes you need.

http://mtbtandems.com/

Scroll down to the Middleburn info section and open it. This is quality stuff at a reasonable cost. The package price includes chainrings. If Middleburn is true on their word, they now offer 130mm spiders also. So this could be a quality, lightweight road alternative for some teams.

http://www.middleburn.co.uk/cranks_tandem.php

PK