Tried it out: Looks like you're right Gavino. MP_Pipeline.dll starts up each new process as an instance of MP_Pipeline.dll.slave.exe, which executes the relevant segment of the script via its own instance of AviSynth. Those processes don't appear (at present) to have any way of receiving input from the parent instance of AviSynth other than the script section itself. So, the AviSource (or equivalent) call has to be in the script section that's passed to MP_Pipeline.
A consequence of that is that if there's any common processing that needs to be done to the video before the processing paths diverge (e.g., in my usual cases, denoising and resizing the picture to fit half the screen width), that pre-processing will have to be run twice... Unless you prepare a mezzanine file first with a separate script.
Common *post*-processing, on the other hand is easier: Just put it after the calls to MP_Pipeline.
SAPikachu, can you confirm/critique that analysis?
It works though! I did a test using an MVTools-based frame-doubler, as a simplified proxy for my 3D processing, and a dummy "minimal load encoder" - i.e. just cropping off most of the picture in VirtualDub and saving small rectangle of it, uncompressed. Using MP_Pipeline, instead of the same script without, finished in 58s rather than 95s. During processing, two processor cores were nearly fully used, rather than one.