It is hard to tell for sure but on the higher volume models of most makers the standard tandem hub still seems to be 145mm spacing. Sure there are some race oriented tandems for sale at 130 and 135 but I have not seen any 150 downhill hub spacing being offered for sale. I suspect that even the outlier Santana sells more 160mm tandems than all the 130 mm or 135mm tandems combined. 130mm is the soon to be dead standard since 11 speed is really larger than 130mm.

I think the trend is away from adherence to broad standards in low volume niche bikes like tandems. The internet brings demand for niche products together making it worthwhile for small manufacturers to sell parts around the world. All sorts of specialty parts are now available that could never be sold through brick and mortar retailers due to distribution and inventory costs.