In the spirit of the traditional 3ds Max workflow for this sort of thing, we have a Krakatoa Delete modifier that respects the “Selection” channel of the particles. Here’s some documentation on how you might use it: http://www.thinkboxsoftware.com/krak-culling-particles-using-s/. You can use the magnitude of the velocity to convert to a selection, then delete the selected particles. It even supports a soft selection delete where each particle is deleted once its selection weight crosses a per-particle random threshold.
This is best accomplished at the moment by duplicating your PRT Loader and inverting your selection weights (ie. 1 - Selection) on one of them so that the opposite particles are deleted in each.