How do you gauge success with mixing oils? If the engine doesn't immediately blow up? Unless you can evaluate the results scientifically, chemically, how do you know you have gained anything? More important, you may be losing versus just going with a single formulation. I thinking mixing oil often results in non-optimal results that aren't always apparent early on. Anybody that crows about it is blowing smoke because they can't prove any benefit over a single oil... except that the engine didn't blow up and maybe a decent UOA (which would have likely been just as good or better with either of the oils alone).
BUT, if you are going to mix oil, the best likely results (or perhaps the "least bad") come from choosing two with similar chemistries. The PYB and Shell combo likely meet this criteria, both being SOPUS products. But again, why mix when there are so many good, synergistically (not haphazardly) formulated products out there.
IMO, mixing sans data is for lawnmower and beaters.... and even then?????