Epoxy Geode

I am trying to figure out how to maintain a nice color separation between two distinct colors during an epoxy pour. I want to create a sliced geode look where the colors meet at a line. So far I can get the epoxy to meet but then the color begins to intermingle which I do not want. I have tried letting the epoxy sit in the cup before i pour in order to reduce the fuildity. I have not used resin obsessions epoxy yet but i am wondering now if different epoxies may yield a different result.