Tachometer feeds from coilpack

[XeonXR6]

Hey there guys, I am using a 4 cylinder tachometer, but my car's ECU only outputs enough signals for a 2 cylinder. I have taken a second feed directly from my coil-pack and what I want to do is use both feeds so that twice the number of signals are send to the tachometer.

I have tried simply joining the two wires together, and hooking it up to the tachometer. The car is having none of this, and when the wires are connected it fires on all 4 cylinders at the same time.

Is there a simpler way of doing this that I have not grasped? I did have a brief thought about using a couple of transistors, but I couldn't see it working.

Any help with this would be greatly appreciated!! Ford Fair coming up soon!

Cheers, Andrew.