The trick is this:
In your form, place two QWidgets, one for each of the pannels. Put those two widgets in a horizontal layout. now, set the size policies of the widgets, so that the left one is fixed, and the right one is minimumExpanding. Next, put the contents of your form sides inside these widgets, and again apply a layout to each of the created "panels". Note that you can also use some other widget (like QFrame or QGroupbox) instead of QWidget if that suits your needs better.