Many people believe the Holy Grail of secure isolation is to use two or more physically separate machines. This belief seems so natural, that we often don't give it much thought. After all, what better isolation could we possible get than physical "airgap"?
I argue with this point of view in this new paper.
I think a good place for in-depth technical discussions around the topics discussed in the paper would be our qubes-devel mailing list.