Paul, Is this second battery also house or is it starter. If this is also the house bank and is left on you really need a independent charger. You could of course attach the charger to a battery combiner but with some of those the batteries will never get fully charged due to loss through the combiner. If you look at the cost of the multi-bank marine
chargers you might consider buying
a second Iota of the same size and connect it to the second battery and run them both simultaneously. This also gives you a back-up charger should one fail.