These are my 2 fave systems. But I would never try to mix them. IMHO, they’re just too big and different from each other. I’ve had clients who wanted this type of Frankensteining with other non-MODx systems, and in my experience it has always been a total cluster$%^$%^. An excellent PHP coder might be able to pull it off... that’s not me. It’s tempting to try it when you have 2 systems with non-overlapping features. I suggest you resist that temptation.
Instead, if I were doing this, I would start with Wordpress, and get that looking good. Then, because MODx is easier to template, I would make a simple template that used the header and footer of the Wordpress theme. Finally, I would make links to the MODx site from WP. Jury-rigged, to be sure.

You could also do the opposite. I’ve templated both quite a lot, and though theming WP is not hard, MODx is even easier.
You’ve been warned. Maybe someone else will have another idea.