actually neither...
I mix in silicone muffin pan cups and they happen to be the perfect mold for pendant blanks. When i pour, there is always a little bit of resin in the bottom of the cup. If i want a regular pendant, I just leave a bit extra in the bottom of the cup and let it set up. If i want a double sideded pendant, i leave about half as much and let that set up, then i pour again right on top of the half blank. the new color attaches right to the old color. If these pieces had to have some structural integrity, I wouldn't do it this way, but they just kinda hang around so i figure no harm no foul. I have sold alot of them and have had no complaints!
when i mix, I can keep the colors seperate somewhat, but not to this extreme. If i let the resin start to gel before i pour, the colors have sharp seperations, or if i pour as soon as i mix in the MEK, the colors blend alot more, but neither have worked for me when trying to keep the colors seperate in this fashion.