We just started building our EIS harness for the G3X the other day, and yup - just one RPM input. I THINK I remember reading that the P-Mag puts out RPM with the ignition "OFF" however (because it is still powered - not sure I remember that right, someone can correct me.
On my GRT (and mag) - equipped -8, I only have the RPM signal coming from one mag, and it has never been an issue - yes, the RPM goes to zero on the right mag, but it has been my experience with mags of the decades that a real problem (with the mag or a fouled plug) is painfully obvious by ear.
Paul