A rejected takeoff is more appropriate to a situation in which going airborne is unsafe; an engine fire, an engine failure, loss of directional control or other situations which affect safety of flight.
Two functioning engines at takeoff power don't create an unsafe takeoff condition, but certainly do create an unsafe stopping condition. If you wish to reject the takeoff, even at lower speeds, there should generally be a very good reason. As one is approaching Vr with a stuck (or in this case broken) throttle/power lever and two good, functioning engines, it's the time to get airborne, safely dispose of the load, and then return.
I've flown a variety of air tanker equipment, and universally we always remind pilots that dumping retardant during a rejected takeoff can create far more problems than it usually solves, for reasons previously mentioned. This is particularly true on older airplanes with less than effective expander-tube brakes.
We had a propeller roll back to about 1,200 RPM at takeoff power in Florida on an active fire, once. It turned out to be a short in the wiring in the stephead motor for the old hamilton standard hydromatic prop. During the roll, as I saw it roll back, I attempted to push it up with the prop switch, no effect. I attempted to move the switch the other way, to no avail. As we crossed the departure end and slightly airborne we then jettisoned the load and shortly thereafter shut down and feathered the #1 engine. With that accomplished, we were able to enter a low downwind and perform an engine-out landing...something for which we all train regularly, and which permits a full runway ahead on which to stop.