A two fly setup (also called a tandem fly or double fly rig) is two flies on one leader: a point fly at the end of your tippet and a dropper tied above it or from the bend of that hook.
That’s the whole trick. When you’re first learning how to fly fish, you start with one fly. Adding a second doesn’t take a special rod or a new cast. It gives the fish two looks, two depths, or two patterns while you figure out what they want.