One would be quite enough.

In fact, Qubes is using hardware virtualization IOMMU/VT-d [0], which has been escaped only once in 2006 by the project founder [1].

[0] https://www.qubes-os.org/doc/architecture/

[1] https://en.wikipedia.org/wiki/Blue_Pill_(software)

I trust Joanna Rutkowska's competence, but I wouldn't bet too much on chip makers not messing up again in the future.

It will be progressively harder, but it will happen.