multi_log.rb

require 'logger'

class MultiLog
  def initialize(*loggers)
    @loggers = loggers
  end

  def write(message)
    @loggers.map { |log| log.write(message) }
  end

  def close
    @loggers.map(&:close)
  end
end

multi_log = MultiLog.new(STDOUT, File.open('debug.log', 'a'))
logger    = Logger.new(multi_log)

logger.warn "hi"