split charge diode - fitting advice

[Oldiebut goodie]

'I'm not connecting the two batteries. Each + terminal from the two batteries would be connected to the rear of the starter motor. Not to each other.'

Via the starter!

I think that this is too complicated for you.
The best advice is that you get someone who understands electrics to do it for you.

[avant-garde]

Because the starter battery would still be connected to the alternator via the starter cable bypassing one of the diodes.

I don't understand this 'bypassing one of the diodes' comment.

So the rough diagram (where the two batteries are as you say connected) was what I thought bigherb was trying to say.

The initial way before I was told not to by bigherb was:
"...there are two red wires going to the starter motor from the alternator. Instead of disconnecting these two red wires coming out of the alternator can I simply add some thick gauge wire to the back of the starter motor onto the same stud that these two red wires are (still) attached to?

That to me is what the circuit shows ( apart from the fact that the alternator is connected to the starter motor)

Hope that makes sense