Jason,
Given that the driving signal is at a common mode of 12 V, the customer certainly can use the INA3221 here and only use a single channel if they wish to. That said, it is still difficult to simply spit out values for this design without having a better picture of exactly what the customer needs here.
The INA3221 has a wide ADC dynamic range, and the customer will need to decide where inside this range they wish to operate. Given that the upper range of this application is 80A, a shunt resistor of sufficient size will need to be chosen to ensure that the customer can handle the thermals generated by the shunt resistor while also delivering the accuracy they need for their application.
As an example, the customer could choose a 500 microohm shunt, but this would still dissipate 3.2 W in the shunt at maximum current. Is that an acceptable amount of heat for their application? At 80 A, the generated voltage read by the ADC at the inputs would be 40 mV, which is well above the offset voltage, and would keep the total error generated below 2% down to around 10 A, where the offset will begin to dominate the measurement. See the output from the INA error tool below(you can find this on the INA3221 home page):
You can use this tool to explore other various values of shunts, but we need to better understand the specs of the customer's system before we can make a rigid recommendation of what value of shunt to use here.
