I know you mentioned using knitting needles for the cores Jake and the aluminum ones cut to length are just the ticket. Be sure to put a dab of worm oil on a paper towel and run the rods thru that oily part before setting them in the mold cavities. Do it each time. Not only does it help in pulling the rods from the baits but it helps the incoming plastic slip over the surface of the rod. The trace of oil left inside the bait will also allow the second color to flow easier thru the core.
We all credit Efishnc, a member here, with the advent of the Ecore style of core production. Lots of twists and turns have evolved since he started the craze. Today just about any mold can be done with a core. I play with the core stuff some but others here do way more work using the technique so keep asking questions and those who "know" will help you out.