There's no harm in running two chargers in parallel. They will either share whatever current the battery will accept, or one will contribute little to nothing, while the other carries the load. The "boiling" is likely a result of too high a voltage coming out of your on-board converter.
Power up the converter while disconnected from the battery, and check it's output voltage. Should be in the 13.6v range, or whatever is listed as it's "nominal" output voltage. Then connect it to a slightly depleted battery and observe the voltage. Should be no more than 14.4 - 14.8v (the converter's bulk voltage setting).
Post the make and model of your equipment... battery, charger, converter-charger for more help.