Just accomplished this same feat a few days ago to go D1 to Barcelona to catch a cruise. i found it most simple to book each independently, and worry about sitting next to each other later.since outbound is most likely a night trip, there isnt much to do after dinner besides read a book ,watch a movie or get some sleep. The only other hurddle ( for me anyway) is my wife tends to like us on one itineraryto make the check in process easier. I told her for 128 K round trip and a Mediterranean cruise, she would just have to get over it