The idea of a shared equipment operation (if you could get a through train together) strikes me as implausible unless VIA and Amtrak somehow go in on an order together. I'm imagining such a service being some absurd mix such as Budds and Amfleets (which sounds like some railfan version of Chutes and Ladders) or Rens and Horizons (which sounds like a disaster area).
There was a short period when the International operated with one Amtrak consist and one VIA consist with no mixing of equipment. It is just that on alternate days alternate consists were used. This BTW is not terribly uncommon in other international services in other parts of the world either.
you can almost rest assured that no one is going to go out and buy new equipment just to provide one daily international service. Ain't gonna happen and is completely unnecessary too.
So I'd suggest that what you imagining is where the problem lies