Did you know, that ... (Part 36)

.. "The Coven" still untaps in your predator's untap phase after you used it a second time in your turn with "House of Sorrow" and passed it to your predator?

This is simply because the cardtext of "House of Sorrow" clearly states that the card untapped by "House of Sorrow" doesn't tap in your (i.e. not your predator's) next untap phase. If you're not in two player game, this is still to your advantage since you will get "The Coven" back after your next untap phase. This way you can effectively use "The Coven" twice during each of your turns without suffering the disadvantage that the card doesn't untap next time you control it.

Reference: VtES Usenet Newsgroup