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Add one of them in the normal way to create a book entry in Calibre.
Having done that find the entry, and select the Edit Metadata option. This will then open a dialog that (amongst other things) allows you to add/remove formats for that book entry.
It is also possible to create the inital entry with no format using the "Add Empty Book" (accessed via down arrow on Add Books icon) option and then add both formats via the Edit Metadata dialog.
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