Which ever coil has the points closed on with engine not running but key is on will get hot. Had an old car once that was not starting and we gave up on it for the day then next morning went out to try an start battery was dead opened hood to put it on charger. That was when we found a large rats nest of fine wire. Seems the key had been left one and the coil got so hot it actually exploded!
Just remember if the coil has a primary resistance of say 4 ohms and the battery is putting out 12 volts it will be drawing 3 amps and those 3 amps will convert to 36 watts of power, in this case heat. So imagine wrapping a 36 watt light bulb in epoxy insulation and it will get hot!