I am actually a firm believer that if Sam and Frodo were to ever marry Frodo would absolutely insist on jointing surnames and put Sam's surname first, for two very clear reasons:
1) He wants to show that Sam's "simple" surname is no less important than Frodo's "gentlehobbit" one, and he knows Sam would refuse dropping "Baggins" altogether
2) Otherwise his initial would be first shortened to S.B, since second surname isn't always written out and the Bagginses have long decided no one with these initials is allowed in Bag End.
















