I suspect that the IRC considers the king stud to carry all the lateral wind load since it acts as a pinned beam form the top plate to the bottom, thereby simplifying the load path and design. The connection from the header to the jack acts as a hinge point so the jack is typically not considered to resist wind loading. I have two arguments against splitting the jack. The first is that though you are correct about the perpendicular grain resistance being lower than parallel to grain. Your comparison to the top and bottom plates is erroneous. The perpendicular to grain resistance depends on how close the applied load is to the edge of a board. When the compression is right at the unsupported edge of the sill there is a higher tendency to crush than say if the load were applied away from the edge. The second argument is one of durability. I would argue that the window sill has a somewhat higher potential for rot than the sole plate so by splitting the jack and placing the end grain of the jack stud ( a structural member) in the plane of higher rot potential is less desirable location than running it to the bottom plate.
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