No discount is given when you book consecutive voyages.
However when booking a logical voyage, which consists of two consecutive cruises sold as a single voyage, with one booking reference (usually with a letter at the end of the cruise number (e.g. M123A) you can typically expect a saving compared to booking the two voyages individually.
If you were to book two or more consecutive voyages not classed as logical voyages, no discount is given. An air credit may be applicable when booking two consecutive fly cruises if one of the flights is not needed. For example, if you booked two consecutive Mediterranean fly cruises, the flight that would bring you home after the initial cruise wouldn't be needed, so a deduction from your fare might be applicable.