Well like I said, the one that the alternator goes to is probably the 'main one'. Toyota would be unlikely to re route the original cabling to add in a battery. Pretty sure it's the driver's side that's original.
Sorry Rich. Note to self. I meant Intervolt. I was trying too hard to be helpful.
Yes there is a reason why not to just use a VSR between the two batteries for the reasons I outlined I guess. The alternator can't supply two different batteries differently. All a split charge does is make two batteries one big averaged battery. A voltage sensing relay is perfect for triggering stuff. I use them, but I'm only going to say this once more, you need something more intelligent wrt charging two different use batteries or one of them at least will suffer. If you use your leisure battery over 3 days in the bush, but don't start your engine once, you now have two different types of battery in two different states of discharge and one charge supply. Fit a proper DC2DC system to look after the second battery. Not doing so has been the death of many an expensive battery. There will always be someone who comes along and says I have a split charge and my batteries have been fine for thirty years. Well better to be born lucky than rich, eh?
Good analogy Chris.It's like sticking two things in the oven at once. The cake needs 140 degrees and the Yorkshire Pudding needs 200. You can't have both temperatures at the same time. Personally, I'd bin the cake.