No, it's not possible, but you can create a custom issue link type like "has subtask" / "is subtask of" and link your subtasks with it. This way you can build an parallel subtask hierarchy.
You can use "Condition / Validation on linked issues" and "Boolean condition / validator with math, date-time or text-string terms" of JIRA Workflow Toolbox to give consistency to this hierarchy and your workflows. For example, you can:
Atlassian Summit is an excellent opportunity for in-person support, training, and networking.Learn more
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG