Basically what you are trying should work.
What jboss version?
Are there any classes in both jars? (this will make it not work)
Are the classes in the 2 jars in the same package? (this should be ok, but might indicate where the problem is).
Can you supply a simple test demonstrating this?
If you don't have duplicated classes please file a bug report with as much info as you can.
I still see this problem in JBoss v3.2.1
Please give me some clues if anybody knows the fix.