I use a 3-fly rig for trout fishing. I used to use a 2-fly rig for steelhead. The bigger fly was always on the end. Unlike what others are saying in here.

However, I no longer use two flies for steelhead. If you catch a big fish that is on fire when trying to bring him to hand. It only takes one time when that second fly gets impaled in your arm to make you stop.

Besides, I don't like the idea of hooking the fish with one fly and the other gets stuck in his eye ball as he fights.