Remember, the normal manifold is part of the figure 8 airflow path. Air leaving one side powers the turbo for the other side's boost. I think it is this way as a method of positive reinforcement. If everything is going smoothly then it will work, otherwise it could produce uneven boost. I'm not a physicist but I'm sure that's not a good idea. Just food for thought when thinking of the single TB and 6runner manifold.