In that case, watch to see if the power pack gets hot when both engines are running. Another indication of overload is if the 2 engines run slower at the same throttle setting than they do if just one is on the track. Finally, if the power pack shuts down or a circuit breaker pops is the obvious sign.
I'm assuming you know that you cannot control the 2 engines separately. They will both go in the same direction and may or may not go about the same speed. To hook the engines together for double-heading, test them running separately on the same track to see if they go at somewhat close to the same speed on the same voltage. If they do, put the slower one in front, and you are good to go (assuming no overloads).
To control engines separately, you need to wire your layout for DC block control, or switch to DCC. And you cannot under any circumstances hook 2 power packs to the same section of track. The result will most likely be a fried power pack, and an out of control loco/train.
If your power pack is overloading with 2 engines, you need a bigger/better power pack. MRC Tech 4 series (used, working Tech 2s or 3s are also very good) power packs have plenty of power for 2 N scale locomotives at the same time, and are much higher quality than the train set packs.
hope this helps