Just run a 2nd battery from a charge-light (fuel pump) relay or high-volt-sense or similar circuit.
Or run a low-volt cutout.
I've posted similar solutions is the past days so please search...
(It's late & I'm tired. And I've plugged and repeated enough..!)
PS - HiJackZX1 - why run a second alternator?
I doubt that your alternator won't handle the extra load.
And if you do run a 2nd alternator, you cannot connect it to your other (standard) alternator system and battery - at least not without special design.
A standard alternator should be able to handle a second battery as per my other posts.
A second battery is the way to go - do whatever with it but you can always start the vehicle as normal.
As to it's sizing, that's the old AH sizing issue. (IE - what load & how many hours until low-volts cutout else load failure?)