I am able make the second megamenu work if I use a new section to create the element dynamically like it is described in the doc. But I want to align the second megamenu to the right of the main one as well as the off canvas toggle. I can make it work with all the items in a Bootstrap row, but things don't align properly vertically so I have to add extra class to each cell of the row (d-flex, align-self-center...). However, that messes up the Layout Settings interface and I don't have access to the cell sizing in responsive mod anymore.
Long story short, the layout settings interface in this case is just not as flexible as I need, and it would be a lot simpler to take care of it directly in a template (block) file. However, I cannot figure out what the jdoc statement for the second megamenu should be. And that, as far as I can tell, is not written in the doc.