yeah you can have the same last name even on different accounts. A lot of people do that. I've never seen an ingame marriage before, but I do know a lot of married couples play the game together, my husband and I included. Half our guid is comprimised of couples.
If you and your spouse really wanted to get into that, I would suggest sending out invites to your friends, asking a GM to marry you in some place of your choosing, or have a friend do it, and make sure there is food and alcohol there for all your guests.