The double D is a nice balanced way to transmit torque, and the shaft can be machined by broaching. The remaining shaft is basically a rectangular shaft. Roark gives guidelines on stresses in rectangular shafts. Smax = .25 Syield.
The flats can be defined by 45 deg intercepts; the subtended angle between the corners of the flat is 90 deg. I have used this in plastic shafts, steel shafts driving PM gears, zinc shafts, etc.