Lots of people do B2B cruises. I am willing to bet there are threads on every cruise line forum about doing B2Bs.
Depending on the cruise line, it can be very easy to book B2Bs. Holland America, for example, sells some B2Bs as " Collectors Cruises " and makes it very easy to book. Other cruise lines are regimented about the two Cruises be booked individually.
B2Bs are not really a special item. Any two or more cruises could be set up as B2Bs, or B2B2B2B....