well..I went to the site
and have these questions
if I purchase the S-afc...then I do not need a A/F gauge because that is already in the S-afc?
and next
did you guy see the POWER FC?....man oh man...that thing is...I guess it will have a price of
around 1k (more) and probably not available for the max...
probably for most hondas, the supra, 300zx, 240sx, rx7, 3000GT
but not our max....

 

You can use the second MAF voltage wire to tap into the O2 sensor signal, it will display voltage.

An edited excert from http://www.roadraceengineering.com/newafcsetting.htm

Blue Wire Mod
Huh ? Blue wire mod? This allows you to read Oxygen Sensor Voltage on the AFC display. On the wire harness for the AFC there are some wires that normally are not used. The Blue wire is meant for the second Air Flow Meter on a Nissan 300ZX. You will connect this wire to the O2 sensor wire right at the ECU.

To read the O2 sensor voltage, scroll down to and select the "etc." menu. Select the "Sensor Check". Of the three readings, O2 voltage will be the middle one (#2)