It looks like there's something screwy with the shared transport setup, which should be the default behavior (meaning, all stacks should share the same UDP layer by default). I'll look into this today.
In the meantime, can you try the following workaround? In the jgroups subsystem, add the following to the UDP transport:
<transport type="UDP" ... >
<property name="singleton_name">udp</property>
</transport>