Building a simple coincidence detection setup

Since I'm blind, I need some help describing the physical setup of two GM tubes in a coincidence array. I have two 220mm by 18mm (approx.) Russian GM tubes. I have an HV splitter box with separate left and right channel stereo pulse outputs for each tube that will connect with my computer. My problem is that almost every description of a coincidence detector shows diagrams or pictures. Not much help. I get it that these two GM tubes should be one above the other. How much separation in mm? Also, how much and how thick lead shielding on either side, below, or above?
I'll be using PRA7 software coincidence detection mode directly off the computer's line input instead of a DAQ board.
So, should I stack the tubes a couple of inches or several inches apart? Any special shielding between tubes? I was thinking of constructing a light wood frame to hold the GM tubes, then add lead sheeting as needed.
Any textual recommendations appreciated. Right now, the two tubes are just taped together and lying on the top shelf of my computer hutch.