I agree completely with frank.
i have made swimbaits from resin and it is a grueling process. My first one i made i was so excited, to try out and learned my bait swam sideways. In my opinion i would separate the two piece lures individually. make a mold for each piece as i have tried both ways. Although it may seem more expensive because it requires more silicone, it delivers more consistency than making a one piece mold and then cutting the resin. As far as resins go, i have used Alumilite and Taps plastic so i can only speak on those products. Alumilite is heavier and taps plastic is more buoyant. I keep a record log of how much microballoons, weight, i use when i pour, so i can get a more consistent action.