And I'm a bad nerd for not getting this stuff revved up and re-submitted to the Marketplace. Someday poser will get real particles and make this stuff obsolete. Until then ...

Increase the Depth for each of the sheets of rain. Select the Control for the Rain tool. The for the Depth control increase the Depth to someth8ing like .125 for the entire duration of the animation. That's most easily done with the graph tool. Open the graph and select all the keys for Depth and [Control] + Drag then up to fix the gap between the sheets of rain.