Here is an old discussion on 'double target' Rego spells about a sword following a grog, my proposal for it and the following discussion.
So your problem is best solved by your troupe. Doing it on the forum only with detailed and published rules makes the resulting spell very hard.
The (HoH:TL p.107) Virtues Mutantum Magic or Tethered Magic, if available, make things a lot easier: they allow to pass the control of your Wind at the Back to the back's owner.