I'm doing more and more co-molded (double-shot rubber) IJ parts and multibodies is a great way to go. What I usually do is create the overall surface body and knit it solid. At that point I copy some faces on the solid and give them zero offset. These faces represent my co-mold part. At this point I thicken the offset surface to create my co-molding, but don't merge it**. It then add the necessary draft interface between the co-mold body and the abs body. I then make a copy of it without translating it. I then use the copy to cut away my ABS part before shelling. I then shell it and whola, I have my abs and co-mold pieces.
** This is the reason I convinced SWX to allow you to copy a body without translating it. At one time you could do it, now you get annoying, but necessary warning (Sorry Edward!).
Mark