I cure my two color jig heads at 350 for thirty minutes, in a toaster oven, and don't have any issues.
Maybe your second powder should be cured at a lower temp, so you might try curing it at 250 or 300 for thirty minutes, and see if that helps.
You might also be getting too much powder on the tap-on side, and that might be shrinking as it cures, causing the lines in the tap coat.
A call to the powder manuf. might also help, since they would know the quirks of each color they sell.