I tried running emerge -f kde on one console, and then emerge kde on another, so that I could utilize both my net connection and CPU resources at the same time, and finish the build quicker. This seems to cause problems. See my forum post that I have linked to.
Yes... This is a bad idea. I'm rewriting my threading code for portage to handle what you're trying to do. I HIGHLY recommend against it as you can have two programs writing to the same files at the same time. Bad news.
Watch bug 1661 for better ways to do this. *** This bug has been marked as a duplicate of 1661 ***