Photoshop :: Grayscale Separation

I need help with separating a grayscale image into dark and light channels. Specifically what I am trying to do is take a grayscale image and divide it into two (or more) channels representing the lower and upper percentages of black. I am doing this so that I can create a black and light gray spot color separation. In screen printing, the lower values of gray tend not to come out and get dropped when burning the film onto the screen. Additionally with dot gain being what it is in screen printing (30%+), it is advantagous to split the gray channel into two channels with only the upper half (50% or more black) into one channel, and the lower half (less then 50% black) into another channel. I have tried to eliminate both upper and lower halfs without altering the other half but can't figure it out. To explain another way, imagine completely elimimating 50% black or more from the histogram without altering the 49% and lower values or compressing them, and then again in reverse, keeping the 50%+ and completely eliminating the lower half.