At both my previous and current homes, the soil is really sandy in many places and it just won’t carry enough current to complete the circuit and provide a zap. I did an internet search and found a suggestion to run two wires in an arrangement where the horse will touch both at the same time. One wire connects to the live wire terminal on the charger and one to the ground terminal. It worked well.
In between houses, I briefly rented a place that was almost swamp. The water table was very close to the surface. I put up the same fence charger I had used at the old place and hooked it to a temporary fence made from electrical tape fencing on step-in fiberglass rods with the conventional one charged wire-ground rod configuration. The young mule nearly levitated when he touched the tape for the first time. The old mule said, "Unh unh, I’ve seen that shit before, I’m not coming within 3 feet of it. 
OK, I googled and found an explanation of what I was talking about with the 2 wire system.
Two Wire System
A two wire ground system should be used if there are more than three strands of wire on the fence. The two strand system allows the animal to complete the circuit by touching a charged wire and a ground wire at the same time. The strands on the fence must alternate between a ground wire and a charged wire: one charged, one ground, and then charged, etc.
If you have a large area that needs to be fenced in, this system is ideal. It is also great to contain animals with long hair, or wool, and areas with sandy or rocky soil. Grounding this system is simple. The wires that are grounded will connect directly to the grounding rod or grounding terminal on the charger. The charged wires are connected to the terminal on the charger.
http://www.zarebasystems.com/learning-center/installation-guide/ground-rod-installation