As far as I'm aware you can. It's just a square wave signal and it's not meant to carry power, just voltage level. Pretty sure tachs would be high impedance devices that have large input resistance so they don't have a big power draw. I'm sure the output line is limited in what it can supply though, so if you stacked enough things together I could see it potentially being a problem. I don't think you'd have issue running two things though.
