A compound harmonic drive, comprising: a compound flex spline comprising: a first internal ring gear, wherein the first internal ring gear is hoop shaped such that the first internal ring gear is open on a first side of the first internal ring gear and open on a second side of the first internal ring gear; a second internal ring gear, the second internal ring gear being coaxial to the first internal ring gear, wherein the second internal ring gear is hoop shaped such that the second internal ring gear is open on a first side of the second internal ring gear and open on a second side of the second internal ring gear, and wherein the second side of the first internal ring gear abuts the first side of the second internal ring gear; a first external gear located within the first internal ring gear and coaxial to the first internal ring gear, the first external gear meshes with the first internal ring gear; and a second external gear located within the second internal ring gear and coaxial to the second internal ring gear, the second external gear meshes with the second internal ring gear, wherein the first internal ring gear and the second internal ring gear are composed of a flexible material, a wave generator disposed radially outward from the first internal ring gear and the second internal ring gear and engaged with both the first internal ring gear and the second internal ring gear; and a bearing element radially interposed between the harmonic wave generator and the first internal ring gear and the second internal ring gear, wherein the first internal ring gear and the second internal ring gear rotate in response to interaction between the wave generator, the first internal ring gear, the second internal ring gear, the first external gear, and the second external gear when the wave generator rotates; wherein the first internal ring gear and the second internal ring gear are connected or integrated such that the first internal ring gear and the second internal ring gear rotate together at the same rate and do not rotate relative each other; wherein the first external gear includes radially-outward-extending teeth that mesh with radially-inward-extending teeth of the first internal ring gear, and wherein the second external gear includes radially-outward-extending teeth that mesh with radially-inward-extending teeth of the second internal ring gear. a compound flex spline comprising: a first internal ring gear, wherein the first internal ring gear is hoop shaped such that the first internal ring gear is open on a first side of the first internal ring gear and open on a second side of the first internal ring gear; a second internal ring gear, the second internal ring gear being coaxial to the first internal ring gear, wherein the second internal ring gear is hoop shaped such that the second internal ring gear is open on a first side of the second internal ring gear and open on a second side of the second internal ring gear, and wherein the second side of the first internal ring gear abuts the first side of the second internal ring gear; a first internal ring gear, wherein the first internal ring gear is hoop shaped such that the first internal ring gear is open on a first side of the first internal ring gear and open on a second side of the first internal ring gear; a second internal ring gear, the second internal ring gear being coaxial to the first internal ring gear, wherein the second internal ring gear is hoop shaped such that the second internal ring gear is open on a first side of the second internal ring gear and open on a second side of the second internal ring gear, and wherein the second side of the first internal ring gear abuts the first side of the second internal ring gear; a first external gear located within the first internal ring gear and coaxial to the first internal ring gear, the first external gear meshes with the first internal ring gear; and a second external gear located within the second internal ring gear and coaxial to the second internal ring gear, the second external gear meshes with the second internal ring gear, wherein the first internal ring gear and the second internal ring gear are composed of a flexible material, a wave generator disposed radially outward from the first internal ring gear and the second internal ring gear and engaged with both the first internal ring gear and the second internal ring gear; and a bearing element radially interposed between the harmonic wave generator and the first internal ring gear and the second internal ring gear, wherein the first internal ring gear and the second internal ring gear rotate in response to interaction between the wave generator, the first internal ring gear, the second internal ring gear, the first external gear, and the second external gear when the wave generator rotates; wherein the first internal ring gear and the second internal ring gear are connected or integrated such that the first internal ring gear and the second internal ring gear rotate together at the same rate and do not rotate relative each other; wherein the first external gear includes radially-outward-extending teeth that mesh with radially-inward-extending teeth of the first internal ring gear, and wherein the second external gear includes radially-outward-extending teeth that mesh with radially-inward-extending teeth of the second internal ring gear.