I am having really good results with one forward facing sensor and one at each front corner, facing forward at 45 degrees. I also have the timing set for 35 milliseconds between firings. No cross interference, even in a hallway with solid walls. I also tried two forward facing sensor and inference was fierce unless you really staggered the left and right pinging. I was trying to sense the direction of approach of a cross path moving obstacle. The code I am using is based off the newping 15 ping sensor example.