DBMS_PARALLEL_EXECUTE

Hello, I try to use DBMS_PARALLEL_EXECUTE package in 11g RAC enviroment.

Sometimes all jobs spawned by this package run only on one instance in the cluster.

Is there any way how to assign jobs generated by this package to specific instance ? 

For example: I execute task (DBMS_PARALLEL_EXECUTE.RUN_TASK) with parallel level 6, so I want 3 jobs running on instance 1 and 3 jobs running on instance 2.

The only thing I have found is that I can assign main task to a job class and job class can be assigned to a specific service.

But this is not what I need.