Sorry about causing panic, there is NO, repeat NO problem whatsoever with polyglot. It was all my own fault:
There was a typo in the WB options for the shortcut to start Fruit. In stead of having -sUCI for the second engine, it had -fUCI. And even if WB does not startup the second engine, it does interpret the WB options that go with it at WB startup. So starting Fruit as first engine in the oldfashioned way, with -fcp "polyglot fruit.ini" would actually see the -fUCI erroneously tagged to the second engine. So it would think polyglot was a UCI engine, and invoke another polyglot to run it.
So indeed, I have always been running Fruit through two polyglots in series. I guess you can do that, now that polyglot understands UCI at its GUI end.

What an inceadibly stupid mistake, that took me an entire evening of debugging to figure it out.
