I don't think this is possible. In Schedules there is one Subtotal (defined by the Flag icon) and there is one Grand Total (defined by the Sum icon).
You will notice that when you want to activate the Flag icon for another field as well, the other Flag icon disappears. So there can only be one Flag icon active at any time, so there can be only one level of Subtotal.
Would be nice, though. If you create a wish for it, I will vote for it.
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