LN is a double barreled surname. (both names your choice)
SO: FN starts and ends with the same letter. MN is an old fashioned biblical name
SO: FN contains a double letter. MN is Russian and uncommon in English.
DD/DD: FNs are a feminine version of a masculine name. MNs are Shakespeare heroines.
DD: FN is related to night time. MN ends in -lyn or -lin.
DS: FN and MN come from here:
http://www.behindthename.com/names/browse.php?type_gender=1&operator_gender=is&value_gender[]=masculine&type_usage=1&operator_usage=is&value_usage[]=rare&type_popularity=1&operator_popularity=isnot&value_popularity[]=&yearoperator_popularity=is&rankoperator_popularity=isDS: FN is Welsh. MN feels slightly dated (up to you what you consider dated)
DS: FN contains an X. MN means King, Prince, or Lord.
DD: FN is three letters long. MN is an unusual flower name.