There can only be one SoS at a time because it is a role held by a single person and not a rank that is bestowed upon several people at once. Since there are many Senators then that title can be kept after they leave office and once an honorable, always an honorable.

Once she's elected it will be President Hillary Clinton and the honorable Bill Clinton, even though he has held the office of the president in the past.

http://www.formsofaddress.info/former.html#FO003