You can use channels (Monolog 2.4+)
In config.yml :
monolog:
channels: [channel1, channel2]
handlers:
main:
type: stream
path: %kernel.logs_dir%/%kernel.environment%.log
level: debug
channel1:
type: stream
path: %kernel.logs_dir%/channel1.log
channels: [channel1]
In controller :
$this->get('monolog.logger.channel1')->info("...");