Process-color trapping is a bit simpler than spot-color trapping,because it’s usually less critical that process-colored elements have traps, but it can be far harder to figure out exactly what color to make the stroke for a process-colored object. And when you’re talking about trapping two process-colored graduated fills, watch out!
The main thing to keep in mind, however, is that, for each of the process inks, the ink percentage used in the topmost object in any stack of objects always wins – they knock out all percentages of that ink behind them, regardless of any overprinting settings.
Unless, that is, the ink percentage is zero. If, for example, the percentage of cyan used in the fill color of the topmost object in a stack of objects is zero, turning Overprint off makes the path knock out any other cyan in the area covered by the path. Overprinting the fill, in this case, means that the area taken up by the fill disappears from the cyan plates – the percentage of cyan in the next object in the stack shows through the area where the objects overlap.
Another way to think of this is to think of each ink in a process color as behaving like a separate spot ink.