Impedance measurements can be made by using either single or dual channel analysis methods.
Single channel impedance measurements are common on production lines, where a two input audio interface can be used to simultaneously capture acoustic and impedance measurements. An example of this method can be found in your SoundCheck installation folder in Sequences > Loudspeakers > Complete Test.
Though simple to implement, a single channel impedance measurement has some drawbacks. It only measures the current across a reference resistor so any effect of the speaker load on the amplifier is not accounted for. Neither are the cables or connectors that are used. Calibration becomes critical, and any errors are reflected in the impedance measurement.
Another single channel option is to use the built-in impedance reference circuit found in the Listen AmpConnect and SCAmp hardware devices.
The dual channel approach simultaneously measures across a reference resistor as well as the amplifier terminals. Because of this, the amplifier does not require calibration and some effects of the cabling is removed. Dual channel requires two input channels, which might present an obstacle on a production line. This method is more common in R&D where there are less time and hardware limitations.
Listen offers two dual channel impedance measurement example sequences: Post-Processing and Transfer Function. Post-Processing is simpler, but requires a sine wave stimulus. The Transfer Function method works with any test signal but requires the optional Transfer Function module.
Links to the dual channel example sequences are below, and a white paper on practical impedance measurement in SoundCheck is attached.