Thanks for your help, it works !
Here are the details of my test (for those who are interested):
I've created two PARs (test_par and test2_par) and put "com.springsource.org.apache.log4j-1.2.15.jar" each PAR file.
Each PAR contains three bundles. The test_impl and the test2_impl bundle contain different log4.properties (to test that they will be loaded by a different classloader).
Bundles in test_par
In addition I've put two more bundles into test2_par in order to test SLF4J.
Bundles test2_par (with SLF4J)
It worked as well.
Cheers and Thanks again