In this specific case you can fake it.
Since small monkey doesn't cast a shade in the shadow of big monkey ... you can just Z-Combine these two renders.
To fake the shadow casted by the big monkey on surface of the small one you can use Mix node > Multiply the Diffuse Indirection pass. It gives you an illusion - it is correct, but it is not, you are just ...
Depth of field requires a bit of knowledge to make it work to your advantage, as it is the combination of many factors:
Aperture of the iris ($f$ stop)
Depth of field is not an absolute term, but is relative to the size of the final image, and there is also a subjective factor as it is usually defined by "the ...