to answer my own question: not a good idea...
........
why? cause for a regulated 9v power source one needs NOT 11.5v as I wrongly stated (I had read the first IC's data, not the 9v IC's) but 15v
15 v is the minimum...
and 7.5 volts for a 9v battery is almost okay...
I mean, a drained battery I'd define from around 7.5 v...
and two of these would be just nearly okay, perhaps only for some minutes...
okay, let's see 3 drained but usable batteries...
below 7.5 volts...
but how low can we go?
in this article an engineer, Russell McMahon explains that a 9v battery at 6v is dead...
and that it is called "dead for a reason" :)
https://electronics.stackexchange.com/questions/475279/is-it-possible-to-use-a-dead-9v-battery-as-a-5v-power-supplySo, the usage could be like connecting 3 drained down batteries...
still above 6v but below 7.5 volts...
NOT a very great usability range...
not to mention that most pedals use nice PSUs... not batteries... except for fuzzes....
I only leave this post here to save time for some others in the future...
................................................................
keywords: dual power supply dual battery power supply voltage regulated power supply, voltage reulator