I know that lots of guys only run one battery, and as long as you take good care of the battery and replace every year or two at the most, you will most likely be OK...
However, I am a firm believer that two batteries are always better than one. Run one as a starting battery and one as a house battery (for accessories). That way if the kicker doesn't put out enough to keep the house battery topped up, at least then you sill still have a fresh battery to start the main. If you wire it up so that both batteries can operate anything in the boat, then you can alternate use on the batteries from trip to trip so that the batteries both get used somewhat equally over thier life, and if one battery goes kaput on you out there on the water, just flip the switch over to the other battery and you are good to go.
For what its worth I hope that this helps.