Need scriptununer script in workflow validation

[Saira]

I have a parent issue that has two subtasks - suppose task A and task B

I want task B's assignee to not be the same as task A's. Task A has a label, let's call that X.

How do I write a scriptrunner script to restrict transition of task B if task A and B have the same assignee ?

I think the script would look something like this:

issue.parent."subtask with the label".assignee.accountId != issue.assignee.accountId

but I don't know what to put in place of "subtask with the label".