Typically a modular page is really a way to build a single page from multiple horizontal blocks of sub-pages. The modular.html.twig template in Antimatter, simply loops over any modular pages and outputs them in divs. As divs are block level HTML this means they are output in horizontal elements.
You wanting to output 2 subpages as a content area and a sidebar is not ideal for modular pages. You would be better off loading a specific page into a sidebar location.
I was more seeing about using the modular system to create modular pages. Having the actual modules inside of the divs with the id makes it easy to control. I see where I can modify to twig file to make it work but I see you want you mean that it is not the best method.
Thanks for the code snippet. That looks like another great option. One thing I am finding with Grav is that there are many paths to getting something accomplished. The more I familiarize myself with it the more I like it.
That looks quite powerful in itself. I can’t express enough how freeing it feels to be able to make changes to the themes, templates and structure of pages so easily.
Thanks for all the hard work you have poured into Grav and for taking the time still to answer the questions on the forums.