From the xIDE window, select Tools>Options>Editor and select "Multiple Document Interface (MDI) style". Note that this change will not take effect until you restart xIDE.
When you restart xIDE, open a project and open a source file from within this project. xIDE by default will open the files within a maximized window. Click on the maximize button to unmaximize the window. Multiple source files can then be cascaded manually by dragging the windows around the xIDE file viewer pane.