You can have as many QMainWindow instances as you want. A QMainWindow is just a QWidget which provides a menu bar, toolbar, status bar and docking framework. But basically it is just a QWidget so you can have as many as you like.
Normally you only have a single QMainWindow for GUI design reasons since it can be confusing to have multiple main windows. However, when implementing applications like QtAssistant which can open a new instance of themselves then the one process has multiple main windows.
What you suggest should work. The window with the tabs would not need to be a QMainWindow unless you want to also want add a toolbar to that window as well.