Make sure there is adequate belt tension in the bottom slow speed tape section, so that the weight of the sheets does not cause any excess drooping.
Adding an edge turner between the two streams in the slow speed tape section prior to the stacker will also help to split the two piles.
A final way to separate the two streams is to steer them with the upper tapes. You can set the tapes to fan outward from the center slit at the point of overlap through the use of tape guides
