The first example (config1.xml) works, the second (config2.xml) doesn't.

config1.xml

<!-- app/config/config_prod.xml -->
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:monolog="http://symfony.com/schema/dic/monolog"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        http://symfony.com/schema/dic/services/services-1.0.xsd
        http://symfony.com/schema/dic/monolog
        http://symfony.com/schema/dic/monolog/monolog-1.0.xsd"
>
    <monolog:config>
        <monolog:handler name="symfony" type="stream" path="/var/log/symfony.log">
            <monolog:channels type="exclusive">
                <monolog:channel>!doctrine</monolog:channel>
            </monolog:channels>
        </monolog:handler>
    </monolog:config>
</container>

config2.xml

<!-- app/config/config_prod.xml -->
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:monolog="http://symfony.com/schema/dic/monolog"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        http://symfony.com/schema/dic/services/services-1.0.xsd
        http://symfony.com/schema/dic/monolog
        http://symfony.com/schema/dic/monolog/monolog-1.0.xsd"
>
    <monolog:config>
        <monolog:handler name="symfony" type="stream" path="/var/log/symfony.log">
            <monolog:channels type="exclusive">
                <monolog:channel>doctrine</monolog:channel>
            </monolog:channels>
        </monolog:handler>
    </monolog:config>
</container>