I like Nails
Join Date: Nov 2011
Location: Sandhills of NC
I would think a double tenon, or a wider tenon would have the same effect. As long as it is not a bridle joint which would not reduce the amount of twisting, and might crack your post - depending on the .. eh hem.. stress the joint is made to endure.
I don't think either joint would reduce the amount of twisting if the board is green or not dry; if it is going to twist.. it will! So I think I would just use a standard M&T, and use a dry seasoned board.
However, we have a caveat.. It seems to me like you plan on cutting your mortise to the top of the post. Based on this statement, " by reducing the overall height of the tenon." which would imply that you plan on cutting the mortise to the top of the post - not enclosed on all sides -- a bridle joint. (I think). I don't think you should do that. I would enclose the tenon on all four sides.