I have a MainWindow .ui file and a QWidget .ui file, I want to use the latter one
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I'm not sure I understand. You want to use a custom widget inside your main window?
If yes then the way it's done in Qt is via promotion mechanism. You create your main window and the custom widget as separate forms. Then, in your main window, you insert a empty placeholder widget, right click on it and select "promote to", where you declare your custom widget name and header location.
More about it "here":http://qt-project.org/doc/qt-5.0/qtdesigner/designer-using-custom-widgets.html#promoting-widgets