ECU Output: Fuel Injector Drivers
The fuel injectors are tiny solenoids. Solenoids look a lot like ignition coils to a transistor: when you quit driving them, you get a high voltage spike just like an ignition coil, although the levels are not nearly as severe. The net result is that the fuel injector solenoid drivers need a sequence of transistors to drive them, just like the ignition drivers. The only difference is that the final output transistor is not required to be nearly as beefy as the ignition driver. For reference, the injector driver schematics can be found here.
The fuel injector drivers are mapped to the processor output pins as in the following table:
The ECU circuit board supports the ability to drive two injectors per cylinder, which is why they are labeled #11 (front) and #21 (rear). The circuit board is ready to accept parts for a #12 (front #2) and a #22 (rear #2). However: just like the ignition circuits, the secondary injectors are not capable of being fired independently. If you really wanted, you could add the parts for secondary injectors and double the amount of fuel getting squirted. However, there would be a ton of work to modify the maps to make it all function. As things stand, a single injector is all the Rotax V990 needs. Maybe if you stuck a giant honking turbo on it, you might need that much more fuel...