Hello and welcome to CoE, every package tier under Bloodline will not be able to choose a surname and will have to stick with whatever family's surname they were "born" with.
If you and your friend have a Bloodline or above (or bought the name selection pack) then you have the option to create a surname, that being said there is a game mechanic SBS wish to incorporate which is locking family members and being able to give their specific code to friends, if this will happen I would say only one of you should pay for a bloodline or above (if you REALLY want your own personal family name) and then save a family member for them to be "born" into
*Anyone correct me if I am wrong