On the 5S-FE engine the two oxygen sensors straddle the front catalytic converter. You will see sensor 1 poking through the heat shield covering the front cat when you open the hood, up high by the head. You can see sensor 2 down lower near the bottom at the level of the oil pan. It is before the flex section of the front pipe. Getting under the car will allow you to see it better.
The second catalytic converter follows the flex section of the front pipe and is not sensed by the ECM and so will not set the dreaded P0420 DTC. Thus, problems with the second catalytic converter will only show up on your emissions inspection readings as higher NOX and HC readings. Remember this vehicle is a Practically Zero Emissions Vehicle and if everything is working and warmed up, it can get the numbers well below the upper limit.
The reason the manual recommends using the oscilloscope is that you want to visualize both O2 sensor voltages at the same time. If sensor 2 switches coincidental with sensor 1, the catalytic converter is not binding oxygen i.e. the active matrix has lost its ability to remove oxygen from NOX and add it to unburnt hydrocarbons. If you use a high impedence 10Meg DVW you can see the voltage activity of the O2 sensors to verify their ability to respond to the presence or absence of oxygen. However, unless you have two DVMs connected to both sensors at the same time you are not going to be able to ascertain switching coincidence.
It is possible that a more capable Diagnostic Scanner tool which can read real time data can to used to display the ECM’s captured data from the sensors. You might be better off to let a emissions specialty shop diagnose the defective component. They will probably charge you a diagnostic fee if you want to replace the defective component yourself.
Please post back how you correct this problem. Good luck on this.