user.rb

class User
  attr_accessor :first_name, :last_name

  def full_name
    "#{@first_name} #{@last_name}"
  end
 
  def full_name=(full_name)
    @first_name, @last_name = full_name.split(' ')
  end
end

user = User.new
user.full_name = "John Doe"
user.first_name #=> "John"
user.last_name #=> "Doe"