I really don't think you can do it with just infrared and green LEDs. Pulse oximeters use one of two combinations: red and IR, or red and green. The common denominator (and the key to the pulse oximeter, itself) is the red light.
That's why we have the mnemonic "SeXy DARLing", which stands for "at SiX hundred nm wavelength DeoxyHb Absorbs Red Light". At green wavelengths, both oxygenated and deoxygenated hemoglobin absorb equally, and the same is true at 780nm IR. So, IR and green are the wavelengths you use to calibrate red.
My best guess, Apple uses IR for the "any time" pulse because it's invisible, and green for the more accurate "on demand" pulse because it's relatively unobtrusive. Shine a green light into skin and it does not go far. Shine a white light, and you get a red halo. Red is the band that propagates through a human body. If they had red LEDs, the light would diffuse a cm or two, enough to light up the skin all around the watch, forming a "halo".
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