I solved the problems of doubling up on BATTERY CABLES on the terminals by using this Blue Sea Systems 9001e e-Series Battery Switch Selector. This switch uses three standard size lugs on the back so I placed it in the POSITIVE cable path whick allows me to only run one battery cable to each POSITIVE battery terminal using same length cables. Each NEGATIVE Terminal goes directly to frame ground using same length cables.
This switch has four positions OFF-BAT1-BOTH-BAT2 which allows me to connect to either BATTERY or BOTH batteries or none (OFF). This also serves as my BATTERY DISCONNECT switch. If the need arises I can charge one battery from the on-board converter and the other battery from an external charger at same time... This is also great for troubleshooting as you have complete control of which battery is connected or off without removing any battery cables.
This is perfect for installing two +12VDC batteries in parallel... ($35 Amazon.com)