Here is the mistake I mentioned: The 'unsigned int order = 0' is not increased and the providers are not added to the map of visible providers; just the first one.
However, this has nothing to do with my problem of showing both inspectors - I simply did not change the visibility of both to 'visible'.

It is not possible to make both inspectors invisible, since a favorite inspector is always selected. The questions is if we need this lines of code. If so, we should add '++order'.