Ive got an autometer A/F guage on my dash....the green wire (or black from the sensor) is the signal to the ECU. The other 2 wires are the O2 heater. I hooked my guage directly to the harness at the computer...I have had no adverse running condidtions, or problems, and you will not harm anything, all the A/F guage is, is really just a volt meter with LED's instead of nubmers. I took the harness connector apart at the computer and solderd a wire directly to the connector for the guage. Dont try to hack into the green wire. That wire has a braided sheild under the green insulation, and under the braid is more insulation around the actual wire that carries the signal. If you want to splice into the wire, you can (should) only do it at the computer side, or the end under the hood. One other thing, I noticed if i unhook the ground from my guage, to power down the guage, the car will not run right, may be a guage problem....otherwise, thats the easiest way to do it. No need to upgrade the sensor or add a second...your just getting redundent.
Mark