# let's say you have an Account model with first name and last name as its fields.
#
# I would put a method that returns both names like so:
#

class Account < ActiveRecord::Base
  def full_name
    "#{self.first_name} #{self.last_name}"
  end
end

#
# Then, on the autocomplete line, add that method as the source of the text:
#

autocomplete :account, :first_name, :display_value => :full_name

#
# :first_name won't matter because you'll override it later
#

# Then, on your Controller, override get_autocomplete_items to:

class SomeController < ApplicationController
  autocomplete :account, :first_name, :display_value => :full_name

  def get_autocomplete_items(parameters)
    Account.where("first_name LIKE ? OR last_name LIKE ?", "#{parameters[:term]}%").order(:first_name).order(:last_name)
  end
end