How long is a piece of string? Have Orbit had other hubs fail in the same way, anyone else out there had the same experience as you? If not after two identical failures I am afraid I can only suggest you may need to spin more and avoid stamping on the pedals.
I am a heavyweight and match most tandem teams for mass. I used to tour extensively, camping, with trailerbike and daughter on tow as well. Nothing ever came past us downhill so I am sure we tested stuff well beyond design limits as you have.
I have broken many hubs on tandems over the last 50 years including five Hope ones, the cassette carriers crack before the flanges on later hubs (Big uns), post the 80s "tandem" hubs. Hope styopped making parts for them a couple of years ago anyway. If you can find a used 48 big un I have 140, 145 and 160 axles to fit them should you need to source one.
The hubs I have not broken are Hadley, only available at 160mm OLN for Santana, so good for me but not your Dawes. I haven't broken a DT one, but they are "only" 40 spoke. I have Hugi ones that havent broken either despite only 2 pawls, but they again are obsolete and 160mm OLN.
In 2018 I custom built from a new frame my "last" tandem and used NOS parts fitting IMHO the best ever rear hub, a Campag tandem hub, 48 spokes, but limited to 8 speed Campy cassettes. Along with Record 8 speed ergos and Euclid running gear I have a bike I am confident will outlast me and would suit the use you need as well.
In your position stuck with shimano running gear I would try the Synergy, it at least has cold forged flanges so only the engagement ring to worry about, or splash out on a DT 40 hole and have it built properly with Sapim strong spoes, that will take the load well enough.
Good Luck whatever you pop for, these days I envy you the knees for loaded touring of Welsh hills.