This screencast shows the possibility of creating overprinted with Separate +
I have a suggestion to make of this feature a user-friendly feature.
Add in the options dialog color separation table into which you yourself said layers
to merge before the simulated separations and layers of which is an imitation of overprinting.
Layer name || merge || overprinting
Layer 1 || yes ||
Layer 2 || yes || K(C|M|Y)
Layer 2 || ||