I assume you want debug.log
to include INFO as well as DEBUG.
log4j.appender.usual=org.apache.log4j.DailyRollingFileAppender
log4j.appender.usual.DatePattern=${roll.pattern.daily}
log4j.appender.usual.layout=org.apache.log4j.PatternLayout
log4j.appender.usual.layout.ConversionPattern=%d{${datestamp}} [%t] %-5p %C{2} - %m%n
log4j.appender.usual.File=usual.log
log4j.appender.usual.Threshold=INFO
log4j.appender.debug=org.apache.log4j.DailyRollingFileAppender
log4j.appender.debug.DatePattern=${roll.pattern.daily}
log4j.appender.debug.layout=org.apache.log4j.PatternLayout
log4j.appender.debug.layout.ConversionPattern=%d{${datestamp}} [%t] %-5p %C{2} - %m%n
log4j.appender.debug.File=debug.log
log4j.appender.debug.Threshold=DEBUG
log4j.logger.customlogger=DEBUG, usual, debug
The key is the Threshold
filter for the appenders themselves. You can send all the logs to multiple appenders and have them filtered there.
The syntax above might be a touch off though, I'm more used to the XML configuration.