Twins charging 3 Batteries

[birdman]

HI All,
I've got twin 1993 150 2-strokes (TXRR?), each with its own starting battery. I'm adding a house battery and was wondering if I can connect both the AUX outputs to the house battery, or should I use just one of them?
Thanks,
Bird

[boats.net]

Those engines have 25 amp alternators so one engine should be enough. The best way to do it is to install a battery isolator to charge 2 batterys at same time but only drawing from 1 battery.

[birdman]

Thanks for the feedback, but I'm confused.
If the AUX output from one engine is feeding the house battery, why would I need a battery isolator? Would the house battery drain from the start battery when the engine is off? If so, what's the purpose of the AUX output on the engine?

I was going to wire in the house battery according to a diagram from BEP Marine. http://www.bepmarine.com/PDF/718_WIRING.pdf
If the Yamaha alternators cannot be wired parallel, I'd just use the AUX output from one engine. Do you know if the alternators can be connected in parallel?