Not sure about the double miles only on Carnival. I did look at using the cruise points and you could use them for many different cruise lines, but they may have been all CCL owned lines (RCCL, Princess etc..) I don't really remember. I did book a NCL cruise using the crad, let me go back and look at my statement and I will find out.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013