Both headlights burned out tonight... Why?

[old man jimmy]

Now you're opening up a whole new can of worms. First off you have to make up your mind what you what it to do. Their are many ways to rig them. If you want to control them from inside the cab, or under the hood etc. But remember, the more complicated you set them up, the more likely they are to failure. I'd simply run your hot lead from aux. battery through a heavy duty throw switch. Now days they have a bunch of after market switches that do the same thing and cost a lot more. But most use electrical selenoids (like the ford selenoid was talking about) and require a lot of wireing. Yours already is fused. That's that relay (fuse) in picture one. And It's already set up from factory to charge both batteries. So if it were me, I'd simply put an isolation switch on the aux. And call it a day.
Tom