What makes GSX decide to *actually* fuel the airplane OR let the airplane refueling itself is not the "Detect custom aircraft system refueling", but the fact if the airplane is configured to NOT "Show Fuel and Cargo dialog during refueling" in the Customize aircraft page and, the Aerosoft Airbus, like many other 3rd party airplanes, is already pre-configured that way so, GSX is never refueling the airplane.
The only thing the "Detect custom aircraft system refueling" option does, is to signal GSX refueling has STARTED, so it will automatically call a Fuel truck even without explicitly calling it from the menu.
It doesn't do anything more than that so, if multiple trucks are in use, the airplane will "spill" fuel and might even complete refueling during the time between two trucks. That cannot be avoided, unless you either use a Fuel Hydrant (which is used by default on gates with Jetways) or disable the option to use multiple trips, so the tanker will have an "infinite" capacity, and will stay there for the entire refueling, no matter how much fuel you requested.
In order to make it work as in real life, the airplane would need to add some code, to know exactly when GSX is disconnecting the fuel hose, so it could "pause" its own refueling, until the next tanker comes.
And yes, GSX allows developers to do that, using the FSDT_GSX_FUELHOSE_CONNECTED L: variables, documented at Page 69 of the manual, that developers can use to check exactly that, and have their own custom fuel system reacting correctly to the possible usage of multiple fuel tankers.
