Parallel battery charging will never be ideal but with larger wires it can be more than adequate. I agree however that if you change the mfg wiring then you need to be sure that there will be no adverse effects on smart or more modern vehicle charging systems.
To me the Ausie guy is off the wall with those 2 batteries only a few feet apart. Yes he is ideally correct and practically giving questionable advice. As the second battery gets further away like in the TC the charging will take longer and will be less than ideal.
DC to DC chargers like the CTEK are ideal for charging the remote battery but even a 20A unit is pricey.
The ideal battery wiring for parallel batteries is balanced wiring and used for adjacent batteries. But it's impractical for isolated batteries.
