The option is in the same place as it always was (in the Simulation page of the GSX Settings), and nothing has changed. It was never called "multiple trucks", but "multiple trips", and I tried it now, and works with no issues.
Currently, is used only by the large fuel truck, the one with the fuel counter, and of course it requires the fuel loaded to be more than the truck trailer capacity (5000, 7000 or 10000 gallons depending on the model).