Just curious. We gave ds both our surnames (I kept mine for personal reasons and this is something I have always felt strongly about). Despite this, many of my friends still call me by my husband's name!
We understand that ds now has a very long name. I am happy for him to drop whatever part of his name he wants when he's older (including if it is my part). I just felt strongly he should start with both equally. We also grew up in London where long and mixed names were not unusual.