If you are dealing with the inexpensive power pack that comes with a typical train set, it may not have enough power to do the job and might burn out. In that case it isn't really a big loss. If you are running dc you want a good throttle anyway. You can pick up Bachmann Spectrum power pack or a MRC Tech 4 or whatever number they are up to for between $25.00-$50.00 depending on amount of power and the "bells and whistles." Good throttles will have circuit protection built in to keep your son from being able to overload the circuits. If you have one of the better throttles instead of the train set throttle, then it will have overload protection built in and he won't be able to hurt anything running two locomotives.
