This actually HAS to be implemented in the firmware, since most printers don't allow gcode commands that move both extruders at the same time. For example, to print with the right extruder (T0), you usually do something like this
M108 T0 ; switch to right extruder
G1 X1 Y1 Z1 E123 ; some E-movement with right extruder
Most firmwares don't support commands like this
G1 X1 Y1 Z1 A123 B456 ; move both the A and B extruders at the same time??
And even worse, many firmwares are literally written to only compute motion parameters for a single extruder at a time. It would require quite a bit of work in some firmwares to add functionality for moving multiple extruders simultaneously.
So no matter what, it would require firmware changes to support this. You can't do it only through software.
Now if you really wanted to get creative, you may be able to electrically wire the printer so that both extruders are essentially wired together and driven from the same input, but that's a lot of hacking/soldering/reverse engineering to figure out how to do that on the FlashForge control board.