Replacing the 12-volt battery with two 6-volt batteries in series will have no effect on the CO detector.
Do you have / can you get a voltmeter or multimeter? If so, measure the voltage at the detector.
If it is somewhere around 12 volts, then the hookup is OK and the detector must be bad.
If the voltage is around zero, then the hookup is bad - perhaps a blown fuse.
Bill
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