HI Andrei, and for the question you wrote above which I happened to see before you deleted it, to do the actual "duplicate sphere" type case (just as an exercise), one easy way to select the stacked up planes is to first select one by clicking on it, and then run Flow, at the prompt for picking the base surface push Edit > Hide to get that previously selected surface out of the way. Then it's easier to click the second plane as the base surface.

- Michael