Unlike cotton, the shorter staple fibers (yak & cashmere) still have scales along the hair shafts (very small scales, but still there) which help in the spinning process. Mix blends (such as those mentioned above) are easier to spin initially because the different staple lengths help stabilize the shorter ones.
In a yak/merino/silk blend, the merino acts as the middle man to bridge the gap between the staple lengths of yak (very short) and silk (VERY long). Whereas in a yak/merino blend, the merino acts as the long staple length.
For homework, we spun various singles and then created a 2-ply. Sandi asked us to ply as close to class as possible, in order to keep as much active twist in the bobbin, because woolen spun yarns tend to lose twist, even in plying. We were going to do a 3-2 cable ply in class.