Hello, @alx-fs and All,
Assuming that, in Settings > Preferences... > MISC. dialog, the options Enable and Enable MRU behaviour, of the Document Switcher(Ctrl+TAB) part, are set, then, if several tabs exist in a single instance of Notepad++, you can switch between the last two accessed tabs, with the Ctrl + Tab shortcut, … in a similar way you would use Alt + Tab to switch between two instances of N++ !
Best Regards,
guy038