Shareholders of a dual currency with reserve have no incentive to actually use the reserve for its purpose. Since there is also no law that forces them to do it anyway, the reserve model is not applicable as long as the shareholders act as rational agents.

The backing has to be done within the system, and right now I don't see any project (neither NuBits nor BitUSD) which does this in a completely convincing manner. Although at least at NuBits those things are heavily discussed.

In total I agree with Omnivion, that it will be extremely interesting to see how these things turn out. However, I personally would like to see a dual currency succeeding, because they in fact fulfill a need.