It looks to me as if the Mac in the youtube clip above with two forestay has also fitted a backstay as well. I would think that two headsails without some additional backstay support is asking for trouble on a Mac.
The video clip certainly seems to show the Mac going fast with a well slotted pair of headsails that should drive it upwind well but it doesn't look comfortable and I would worry about the mast load the way it was sailing.
I have seen a light small section mast with high loads collapse under shock load in a choppy sea.
Mike C - Tarka 26M