My guess is that yes, they would be sharing a bed while honeymooning/on the voyage, but I've not been able to convince myself of it and it's a somewhat crucial detail. Research done includes googling various permutations of "newlyweds turn of the century" "newlywed sleeping arrangements 1912" "married couples bedrooms turn of the century" etc. etc., as well as an almost silly amount of reading and documentary watching about the Titanic, with particular focus on first-class cabins and young couples traveling first class. Any help or further sources to pursue will be much appreciated!
On a related note, in the course of researching this story I've read more or less the entire corpus of little_details posts tagged 1900s, 1910s, world war i, etc. It has been stunningly enlightening and given me all sorts of little insights into things I wouldn't have thought about. Thanks, everyone!