Yes, the second wire is either GND or +24V. I’d check the user manual for more information.
This can also be determined by measuring the voltage at PB pin against GND. To make button work and light up the LED in in the wall console, there’s difference of potentials between the connections. If PB reads around zero (in technical term: pulled down), then the other pin for the wall switch is +24V. If it reliably reads any stable voltage above 3V (pulled up) then the other terminal is GND (0V).