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>