A two legged stirrups is a U. For example if you lookat a arectangular beam the stirrup would hok on the the bottom left bar go parallel to the left face, have a 90 deg bend over the top left bar, parallel to the top face, another 90 deg bend at the top right bar, parallel to the right face and hook the bottom right bar. There are two vertical legs crossing the shear plane.
A single legged stirrup would look like a c-bar. Straight bar with hooks at either end one around a top bar and one around a bottom bar. There is only one vertical leg crossing the shear plane. Hope that helps.