My .02cents.

Get a quality bike with quality parts. There is a lot of remote riding on this route and you will be well served to have proper equipment. If you are doing it self supported, that will add a lot of weight to the bike and be harder on the bike/components. You will want to get used to riding on a tandem on similar terrain (with the gear that you plan to bring). Riding with additional gear really changes the handling of a tandem. You need to think about how you will carry your gear. Frame bags are the common choice, but you can use panniers (heavier).

I think it sounds like a great adventure, but you should read up on whatever you can so that you are well informed as to what you are getting into.

FWIW, we primarily ride mtb tandems (we have several). If I had to give a list of bikes that might be worthy or this trip, I'd start here.

Mtb Tandems Fandango 29" wheels
Salsa Powderkeg
Co-Motion Java
Ventana (might be overkill to have a full suspension, but would still be fun and is a quality bike)

I'm sure there are a lot of other bikes that would be up to the challenge.

You are a relatively light team, so that is a plus. But remember, don't skimp on quality parts. Things break. Tandems are harder on components than non-tandems and you don't want to be walking 50-100 miles to the nearest town if you can avoid it.

Best of luck!