NEdit is able to display files in distinct editor windows, or to display files under tabs in the same editor window. The Options for controlling the tabbed interface are found under Preferences -> Default Settings -> Tabbed Editing (cf. "Preferences", also "NEdit Command Line").
Notice that you can re-group tabs at any time by detaching and attaching them, or moving them, to other windows. This can be done using the Windows menu, or using the context menu, which pops up when right clicking on a tab.
You can switch to a tab by simply clicking on it, or you can use the keyboard. The default keybindings to switch tabs (which are Ctrl+PageUp/-Down and Alt+Home, see "Keyboard Shortcuts") can be changed using the actions previous_document(), next_document() and last_document().