Very gently and slowly pour the second color on top of the first color. If you pour too quickly, or from too high up, you'll start to "swirl" the colors together and won't get a clean line in between them.
(If you want the colors more swirled, though, pour from high enough so that the second color penetrates into the first and/or swirl through it with a chopstick or rubber spatula.)I wanted a nice clean yin-yang sort of line in this batch, so I poured slowly and carefully.
After all of the soap has been poured, very gently lower the mold so that it's horizontal and wiggle the mold a little to make sure that the soap "settles" level.
If you're paying attention, you'll notice that my purple soap layer has turned brown - to see why, read From Purple to Zen.