So when Piper says that God did not merely foresee or permit the terrorist attacks of 9/11 but designed and governed them and when he says that a tornado was not merely permitted by God but sent by God, he is simply saying what conservative Calvinists (not necessarily all Reformed people) have always said.
Is Olson suggesting that God merely “permitted” the tornado? What’s that supposed to mean? Is he ascribing freewill to the tornado? Is the tornado a sentient being with a mind of its own?
Arminius held to the doctrine of concurrence. Concurrence simply means God allows whatever happens.
Is that all it means? That’s not how Olson used to define it. Here’s what he used to say:
Arminius was puzzled about the accusation that he held corrupt opinions respecting the providence of God, because he went out of his way to affirm it. He even went so far as to say that every human act, including sin, is impossible without God's cooperation! This is simply part of divine concurrence, and Arminius was not willing to regard God as a spectator.
According to this, God does not permit sin as a spectator; God is never in the spectator mode. Rather, God not only allows sin and evil designedly and willingly, although not approvingly or efficaciously, but he cooperates with the creature in sinning without being stained by the guilt of sin.
God both permits and effects a sinful act, such as the rebellion of Adam, because no creature can act apart from God's help. In several of his writings Arminius carefully explained divine concurrence, which is without doubt the most subtle aspect of his doctrine of sovereignty and providence. For him God is the first cause of whatever happens; even a sinful act cannot occur without God as its first cause, because creatures have no ability to act without their Creator, who is their supreme cause for existence...
R. Olson, Arminian Theology: Myths and Realities (IVP 2006), 121-23.
So by Olson’s own definition (summarizing Arminius), God didn’t merely permit the tornado. Rather, God is the ultimate cause of the tornado.
Clearly Calvin understood everything as foreordained and rendered certain by God which, for him, probably also for Piper, does not rule out secondary causes. But secondary cause (which is what I assume you mean by “double effect”) doesn’t get God off the hook. Ultimate responsibility lies with ultimate cause.
In which case, God is ultimately responsible for the tornado, and the resultant fatalities.
But even more: I’d like to hear one of them (Calvinists or anyone who believes God foreordains and designs and renders certain everything that happens) say publicly that it was God who caused a predator to kidnap, torture, rape and murder a child. I seldom hear or read them saying so. And yet, it would seem that, too, must be included in God’s meticulous providence AS IT IS BELIEVED BY THEM.
But according to Olson, God didn’t merely “permit” the assailant to rape, torture, and murder. God “cooperated” with the sadistic, murderous rapist. God "willingly" and "designedly" “helped” the sadistic, murderous rapist. God is the “effecter” of the rapist’s sadistic, murderous deeds. God is the “first cause” or the “supreme cause” of the outcome. And “ultimate responsibility lies with ultimate cause.”