Give this link a read:
http://www.smartgauge.co.uk/batt_con.html Use the most complex Method 3 for a perfect balanced parallel connection of any number of 12V batteries (even though the example photo shows 4 batteries). Use the simpler Method 4 for a perfect balanced parallel connection of four 12V batteries. I use Method 2 for a perfect balanced parallel connection of just two 12V batteries (this method is not perfect for more than 2 batteries, however). The goal is to have all current from every battery flowing the exact same "overall wire path distance" from it's positive terminal, to the load, and from the load back to it's negative terminal .... as every other battery in the bank. Of course all of this wiring should using as large a gauge cabling as you can afford and fit.