tach misreading - showing wrong rpm's

I'm pretty sure you should be able to hook both up together. I believe is just puts out a 12 volt square wave with a frequency that matches RPM. At one point I had the tach line feeding a tach for me and being the RPM signal for my fuel only Megasquirt setup. It doesn't really need to provide any current, so I think it should split okay. Seemed to work well enough for me at least.