Just as in Bonaparte at Marengo artillery cannot shoot in consecutive turns, right? In order to fire artillery twice into the same locale, it must, for example, secure the approach and range at 9:00 am; bombard at 10:00 am; range again at 11:00 am; fire the second time at 12pm. Am I reading this correctly.?
Yes, you are correct.
However unlike Bonaparte at Marengo, in Baptism at Bull Run you have the option of ranging with more than one artillery unit that is securing a particular approach, thereby allowing you to alternate ranging and bombarding with those artillery units and thus, bombard every turn.
You may want to consider reading the sidebar entitled "Range Artillery" on page 18 of the rules, which in addition to reviewing artillery ranging & bombarding also covers the concept of counter-battery fire.