That's what I was thinking I could/would do. With the car on, the batteries wouldn't ever really be at rest, it would be a constant up and down just like normal, so it wouldn't be a problem, it seemed like the only issue came in is when the car was turned off, like you said, they will try and balance each other out.

What's the deal with mixing types/brands of batteries?
Seems like people make a big deal about mixing brands. Is that just back to the different at rest levels? Does the isolating relay fix that problem?
What about mixing types? Example, they say you shouldn't use a red top to start wired to a yellow top for the audio.
Why is that?
again, does that isolator relay fix that problem?
That's a major concern because the nice lightweight batteries that I would be using up front are those quick discharge type, like like a red top. They are smaller, lighter, and still pack a good punch for getting things started.
Can that battery be matched with a deep cycle battery in the back? if they had the relay to isolate them when the car's off? OR do they cause each other problems with the car running too?