That is not correct. In an Advanced Flow, if you connect cards in series (after each other, so not in parallel) then the next card is not executed before the prior completes. That is why you can also run a second flow only after all the flowcards of the first flow have completed. The second flow needs to be started of course from within the first flow to make use of this principle.
As an example: Using flows as subflow - #4 by SunBeech
