Myself I would not recommend an attempt on the TD route with marginal equipment. I rode from Banff to Whitefish last June on my bikepacking rig and the roads and trails aren't all that bad; however, I had a very solid bike with top quality components under me. Those areas are desolate and not worth the risk of equipment breaking.

I am in the process of building a new off road tandem for me and my wife.
The hubs will be Onxy as they now make 12x148 Boost through axle hubs with 36 holes. Not cheap.
The front fork will be a top quality suspension fork. Again, not cheap.
Still looking for cranks but they will not be cheap.
Rims??? Not cheap.

Everything for an off road tandem needs to be built a bit beefier than for a regular bike. The Salsa, Co-Motion and Ventana bikes are all great bikes but not cheap. If you can afford one of them, then that is the route to go.

Myself, we're getting a custom Curtlo built and expect it to be fantastic.