gistfile1.rb

maximum_id = 2

[:customer_a, :customer_b].each_with_index do |database_name, index|

  id_offset = index * maximum_id

  ActiveRecord::Base.connection.execute %{
    insert into people
      select
        `id` + #{id_offset},
        `name`,
        #{database_name}
      from #{database_name}.people
  %}

end