How can I configure Logback to log different levels for a logger to different destinations?

I believe this would be the simplest solution:

<configuration>
    <contextName>selenium-plugin</contextName>
    <!-- Logging configuration -->  
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <Target>System.out</Target>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
            <level>INFO</level>
            <onMatch>ACCEPT</onMatch>
            <onMismatch>DENY</onMismatch>
        </filter>
        <encoder>
            <pattern>[%d{yyyy-MM-dd HH:mm:ss.SSS}] [%level] %msg%n</pattern>
        </encoder>
    </appender>
    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
            <Target>System.err</Target>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
            <level>ERROR</level>
            <onMatch>ACCEPT</onMatch>
            <onMismatch>DENY</onMismatch>
        </filter>
        <encoder> 
            <pattern>[%d{yyyy-MM-dd HH:mm:ss.SSS}] [%level] [%thread] %logger{10} [%file:%line] %msg%n</pattern> 
        </encoder> 
    </appender>
    <root level="INFO">
        <appender-ref ref="STDOUT"/>
        <appender-ref ref="STDERR" />
    </root>
</configuration>

Update: For an all configuration based approach using Groovy see Dean Hiller's answer.

--

You can do some interesting things with Logback filters. The below configuration will only print warn and error messages to stderr, and everything else to stdout.

logback.xml

<appender name="stdout" class="ch.qos.logback.core.ConsoleAppender">
  <target>System.out</target>
  <filter class="com.foo.StdOutFilter" />
   ...
</appender>

<appender name="stderr" class="ch.qos.logback.core.ConsoleAppender">
  <target>System.err</target>
  <filter class="com.foo.ErrOutFilter" />
   ...
</appender>

<logger name="mylogger" level="debug">
    <appender-ref ref="stdout" />
    <appender-ref ref="stderr" />
</logger>

com.foo.StdOutFilter

public class StdOutFilter extends ch.qos.logback.core.filter.AbstractMatcherFilter
{

    @Override
    public FilterReply decide(Object event)
    {
        if (!isStarted())
        {
            return FilterReply.NEUTRAL;
        }

        LoggingEvent loggingEvent = (LoggingEvent) event;

        List<Level> eventsToKeep = Arrays.asList(Level.TRACE, Level.DEBUG, Level.INFO);
        if (eventsToKeep.contains(loggingEvent.getLevel()))
        {
            return FilterReply.NEUTRAL;
        }
        else
        {
            return FilterReply.DENY;
        }
    }

}

com.foo.ErrOutFilter

public class ErrOutFilter extends ch.qos.logback.core.filter.AbstractMatcherFilter
{

    @Override
    public FilterReply decide(Object event)
    {
        if (!isStarted())
        {
            return FilterReply.NEUTRAL;
        }

        LoggingEvent loggingEvent = (LoggingEvent) event;

        List<Level> eventsToKeep = Arrays.asList(Level.WARN, Level.ERROR);
        if (eventsToKeep.contains(loggingEvent.getLevel()))
        {
            return FilterReply.NEUTRAL;
        }
        else
        {
            return FilterReply.DENY;
        }
    }

}

Solution based on configuration only, with a ThresoldFilter and LevelFilters to keep things really simple to understand :

<configuration>
    <appender name="STDERR" class="ch.qos.logback.core.ConsoleAppender">
        <target>System.err</target>
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
          <level>WARN</level>
        </filter>
        <encoder>
            <pattern>%date %level [%thread] %logger %msg%n</pattern>
        </encoder>
    </appender>

    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <target>System.out</target>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>DEBUG</level>
          <onMatch>ACCEPT</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>INFO</level>
          <onMatch>ACCEPT</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>TRACE</level>
          <onMatch>ACCEPT</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>WARN</level>
          <onMatch>DENY</onMatch>
        </filter>
        <filter class="ch.qos.logback.classic.filter.LevelFilter">
          <level>ERROR</level>
          <onMatch>DENY</onMatch>
        </filter>
        <encoder>
            <pattern>%date %level [%thread] %logger %msg%n</pattern>
        </encoder>
    </appender>

    <root level="INFO">
        <appender-ref ref="STDOUT" />
        <appender-ref ref="STDERR" />
    </root>
</configuration>