I gave up searching for anything 40H after searching up and down the internet, and went with a 36H Ryde Andra to make hub selection easier.
According to some tandem websites/forums I scoured at the time, modern rims/spokes/nipples are far far better than years ago and 40H rims are almost unnecessary, even for bigger riders or luggage.
JD Tandems have 40H rims (Alex G6000) but I would agree - 36H trekking/MTB hubs/rims will probably be fine. Find something with bearings in the freehub body as well as the hub shell, that'll make the biggest difference.
I gave up searching for anything 40H after searching up and down the internet, and went with a 36H Ryde Andra to make hub selection easier.
According to some tandem websites/forums I scoured at the time, modern rims/spokes/nipples are far far better than years ago and 40H rims are almost unnecessary, even for bigger riders or luggage.