After talking with a local tandem dealer, I suspect that Seven could build an Evergreen (
https://www.sevencycles.com/bikes/bi...ergreen-sl-007)
that would handle 460# without undue flex. The 650 wheels would be a better option than 700c at that weight and at least 36 spokes. With dual hydraulic discs and
couplers you would likely be north of $12k. Any one building Ti tandems is going to slot them into the custom bike build as the market for these jewels is limited. Your
problem absent experience is knowing what you want in a frame size and configuration. I would be more leery of carbon than Ti, as CF is very tough but when its limits
are exceeded, like glass it breaks. For travel purposes I think a single wheel trailer would be preferable to loading the bike up with front and rear panniers. Depends
on where you are going. Personally I have about 75k miles on 3 Ti frames, so far so good, only problem was with a '70s era Teledyne made from reactor tubing that lasted
30 yrs. Teledyne did NOT know what it was doing and ignored their own engineers advice to use 3.25Al-2.5V alloy, using CP Ti instead.
One other aspect of dual disc brakes is that the frame can be made for tire clearance of 50mm (or more) and for fenders. Gravel bikes have had the option with disk
brakes of either 700c or 650 wheels, with the smaller diameter of the 650 allowing larger tires for more unpaved options.