There are a couple of ways to run multiple engines safely.
1. 1 Powered - 1 Dummy, you'll see this in a lot of E8, E9 AB sets or even 1 Powered 2 dummy ABA sets which looks sweet. One thing to keep in mind though is that only the powered engine usually lights up.
2. DC - Multiple Engines of the same brand/type with different road numbers. These engines will have the same motors and gearing and will produce very close to the same pulling force and speed. They are then cooperating and one's not dragging the other one around the track.
3. DCC - Multiple Engine Consist. This is the coolest way to go, you can have multiple engines from different providers, and adjust their individual speed curves so that they match. You then assign the units to a consist. This allows you to control as many engines as you like as if they were only 1 engine. The lights can synch up, and when you have an ABA configuration, the headlight of the unit that's moving forward will light up. Reverse direction and the former back engine now is the front and it's headlights now operate.
I just ordered in a couple of Atlas DCC units (powered but no sound) to go along with my Proto DCC w/ sound engine. I figured that 3 engines all with sound at once might be a little overkill. I can't believe how loud the little guy is by himself. Really looking forward to starting to run multiple engine units.
3a. One other thing you can do, that is really neat from an operational perspective is to have a steep grade that one engine is not able to pull up a consist of cars. Then you can have a second operator switch onto your track, join up with a helper engine, and then decouple them once you crest the summit.