BVI hunter, I'll take a stab at your questions:
I've only used the TM-808 (but I assume the other 2-box units are similar): It can not differentiate "trash" versus "gold. It simply beeps on metal, regardless of composition. Hence it has no "discrimination" per se (however, nor does
ANY detector differentiate between "trash" and "gold" anyhow. They merely read conductivity, not composition. Just to set the record straight
)
And no, it does not ignore all "surface" targets, unless the surface target is small. The targets it "ignores", is anything smaller than about a tobacco tin. So if a flattened soda can or tobacco tin sized item were just under the surface, yes you would hear those. So it's not that it "ignores" items at a "certain depth", but rather, it's ignoring items based on size, not their depth.
In my opinion, the benefit of the 2-box unit was not so much for the depth it affords. Because when you think of it: SO TOO can a standard detector get a soda can to well over a foot deep. Or a tool-box or hubcab sized item to 2 or 3 ft. or better, blah blah blah. For this reason, a lot of people will suggest simply using a standard detector for cache hunting, which can often-time equally get big objects deeper. And the rationalization is that ..... at least with a standard machine, you can go "back and forth" between coin-sized object hunting, and big object hunting.
HOWEVER, my experience is that if you are truly hunting for a cache, and if you are in an area riddled with regularly occurring household/homestead type debri, then you are much better off with a 2-box unit. NOT because of any supposed depth it adds, but merely so that you are not bedevilled with all the little stuff (you know, pesky individual coins, pulltabs, nails, etc....). Because the 2-box unit simply doesn't even hear individual little targets like that. It only hears the bigger items. So it's sort of the "perfect discriminator" to put your time best spent on the item you are distinctly looking for.
If you try to use a standard machine, and try to tell yourself "I'll simply ignore signals from all small items", it will be an exercise in frustration. Because you'll be constantly trying to mentally evaluate big vs little endlessly. You know, like how a soda can at a foot *might* sound like a quarter at 1", etc... The 2-box unit takes all that guessing game out of it, and you only hear the big items. Ideally soda can sized or larger. But with fine-tuning and close-listening, I've found objects as small as domino sized, or butter-knife and/or tobacco tin sized. But that's the smallest it can be refined to find. Normally (without super tuning) it's not going to find objects smaller than about a soda can.
As for water-proof, I haven't heard of any 2-box machines that are made in a water-proof package.
