2-in-1-out mixing hot end not coming to temp

[leolson]

Hi, all.  I bought a ZSYong 2-in-1-out mixing hot end a while back to replace the stock hot end on my Geeetech A10M.  Got it installed, but it stubbornly refuses to come to temperature.  When I insert a 24v 6x20mm heater cartridge and fire it up to 200c, the temp slowly increases to around 100c and levels out, which is what I would expect to see if I plugged a 24v cartridge into a 12v connector.  However, when I swapped the cartridge for a 12v one and turned it on, the temperature shot up remarkably quickly, overshot by about 40c, then came back down and started oscillating wildly.  Which is the behavior I would expect to see if I plugged a 12v cartridge into a 24v connector.  I have tried multiple cartridges, multiple thermistors, and a number of different TEMP_SENSOR settings in the firmware, and I am no closer to a solution.  Am I missing something?  Has anyone seen this before?  Any help would be greatly appreciated!