Is it possible to assign conditional tasks with Fork node in JBPM.

As per my requirement, workflow should allow optional Parallel tasks and a mandatory sequential task.
But I'm getting some issues while conditionally assign tasks with fork.

Process definition:

<?xml version="1.0" encoding="UTF-8"?>
<process name="conditional_task" version="1" xmlns="http://jbpm.org/4.0/jpdl">
 <start g="27,32,80,40">
 <transition to="Validate Approval Seqence"/>
 </start>
 <!-- Checks which sequence to initiate ie., Either Parallel or Sequential -->
 <decision expr="#{isOptionalTasksExists}" name="Validate Approval Seqence" g="257,31,80,40">
 <transition name="true" to="Assign Parallel Tasks" g="-24,2"/>
 <transition name="false" to="Sequential Task" g="564,54:-75,-21"/>
 </decision>
 <!-- Parallel Task Assignment -->
 <fork name="Assign Parallel Tasks" g="258,139,80,40">
 <transition to="validateFirstParallelTask"/>
 <transition to="validateSecondParallelTask"/>
 </fork>
 <!-- Conditional Task Assignment -->
 <decision name="validateFirstParallelTask" g="135,221,80,40">
 <transition to="First Parallel Task">
 <condition expr="#{firstParallelApprover != null}"/>
 </transition>
 </decision>
 <task assignee="#{firstParallelApprover}" name="First Parallel Task" g="71,325,169,40">
 <transition name="Approve" to="wait for other tasks" g="-45,-18"/>
 </task>
 <decision name="validateSecondParallelTask" g="384,221,80,40">
 <transition to="Second Parallel Task">
 <condition expr="#{secondParallelApprover != null}"/>
 </transition>
 </decision>
 <task assignee="#{secondParallelApprover}" name="Second Parallel Task" g="308,324,204,40">
 <transition name="Approve" to="wait for other tasks" g="-45,-18"/>
 </task>
 <join name="wait for other tasks" g="270,436,80,40">
 <transition to="Sequential Task"/>
 </join>
 <!-- Sequential Task -->
 <task assignee="#{sequentialApprover}" name="Sequential Task" g="494,437,147,40">
 <transition name="Approve" to="Finish" g="-45,-18"/>
 </task>
 <end name="Finish" state="Completed" g="778,431,80,40"/>
</process>

This definition forces me to supply either (2 Parallel and 1 Sequential) or (0 Parallel and 1 Sequential) approvers, but not 1 Parallel and 1 Sequential.

Thanks in Advance