This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Are the classes in the two JAR files in different packages? Then you can just configure your loggers and appenders in the Log4J configuration file differently for the classes in the two different packages. No need to do complicated tricks.