After reading the full text article I was surprised to find the protons from the aluminium foil/picosecond pulse interaction actually come from hydrocarbon impurities that were on the back surface of the foils.
If they don't guard that hydrocarbon film on the back of the aluminium from the Boron-ionizing nanosecond pulse then they don't get protons. So that is why the second thinner aluminium film is there; to shield the impurity layer on the thick aluminium film.
I would like to say that contrary to software engineering, physical engineers tend to use quirks more often than software engineers, but the reality is that in both worlds quirks are heavily used and accepted. Hence MVP and other quick and dirty delivery models are popularized...
If they don't guard that hydrocarbon film on the back of the aluminium from the Boron-ionizing nanosecond pulse then they don't get protons. So that is why the second thinner aluminium film is there; to shield the impurity layer on the thick aluminium film.