Am I being dumb, I can't see a way to do for two paths on Shape Layers,
what the Smart Mask Interpolation does for two Mask Paths.





It is true that you can't use Smart Mask Interpolation on shape paths directly, but what you *can* do is: copy the shape path keyframes, paste them into a mask, use SMI, then link the shape path to this mask path and you will get the SMI behavior in the shape path (or you can copy and paste back into the shape path, but it won't update automatically if you adjust the mask path later).