I Hope this is what you mean. I can't see how to attach a file, but its small, so I'll just paste it here. We have a couple instances in the executions, but it never gets past the first one.. you'll probably have to remove the custom activity ..
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<?xml version="1.0" encoding="UTF-8"?>
<process name="test" xmlns="http://jbpm.org/4.4/jpdl" key="tasktest">
<start g="66,327,48,48" name="start1">
<transition name="to taskForPeterOrMary" to="taskForPeterOrMary" g="-126,-19"/>
</start>
<task g="115,202,138,52" name="taskForPeterOrMary">
<assignment-handler class="wftest.MyAssignmentHandler"/>
<transition name="to fork1" to="fork1" g="-45,-19"/>
</task>
<task g="419,69,92,52" name="taskForPeter">
<assignment-handler class="wftest.MyAssignmentHandler"/>
<transition name="to join1" to="join1" g="-45,-19"/>
</task>
<task g="417,352,92,52" name="taskForMary">
<assignment-handler class="wftest.MyAssignmentHandler"/>
<transition name="to join1" to="join1" g="-45,-19"/>
</task>
<fork name="fork1" g="362,199,48,48">
<transition name="to taskForPeter" to="taskForPeter" g="-88,-19"/>
<transition name="to TaskForMary" to="taskForMary" g="-87,-19"/>
</fork>
<join name="join1" g="549,208,48,48">
<transition name="to custom1" to="customActivity" g="-66,-19"/>
</join>
<custom name="customActivity" g="628,74,92,52" class="wftest.MyExternalActivity">
<transition name="to end1" to="end1" g="-46,-19"/>
</custom>
<end name="end1" g="749,211,48,48"/>
</process>