That's the problem. The one place that has most everything Stronglight is XXCycles, which is haughty and French. I describe the deal with them here. There are a few internet UK retailers (e.g. Ribble), that sell some Stronglight, speak English, and have good prices, but they don't list the Impact tandem crankset. Worth emailing them to se e if they could get you a set.

It is a shame there is no US Distributor, because the Euro types that post on the Weight Weenie forum really like Stronglight stuff. If anyone wants to get involved in the bicycle business, the US Stronglight distributorship is a wide open opportunity!

I think the best thing to do is to email XXCycle, and see if you can get an intelligible response. Then proceed cautiously. The prices are really good, and so if it didn't work out it wouldn't be the end of the world. And it would give you a story to tell when tandem cognoscenti admire your drivetrain.