Good morning old pro: I have, but it wasn't all that deep. As for showing a picture , sorry, I didn't have a camera and the proceeds were quickly converted into Pesos and food, etc..
I did recover some 8 mule loads of 8 REALES from a mule train assault of the 1800's, but it was with a GARDNER with the 3 ft coil. it was only 4 ft deep on the bank of an arroyo in sand. I have posted pictures of my share here in TH..
The two box units definitely have their place, but then not very many deposits are over the depth that one man can dig and still get out of the hoie easily by himself. Most are simply arm depth for ease of recovery later, after all, if one doesn't know of it's existence, it is just as safe at 6" as 6 ft..
Also the deeper a hole is dug, the more impossible it is to hide it. the soil expands as it is excavated, so you have to dispose of the excess left over from the space occupied by the hidden materiel, as well as the excess soil itself. If you evenly spread it around so that it isn't noticeable, then the first rain will cause it to form a depression, so will time due to natural settling. etc. etc. Many treasures have been found by just such signs.
Then again there is the time factor, most caches looked for are the result of a robbery, just how much time would a robber have to dig a deep hole knowing that a pose etc. is hot after him?
The one main feature for a two box that I love, is the ability to search for for an object the size of your fist or larger, in an area of tons of small misc.metal or where a large area has to be covered. It also serves well in a heavily grassed zones, and is fast. It totally ignores bottle caps and pull tabs, hence is useless for individual coin searching, but perfect for the larger deposits.
YES! a two box is an important part of my equipment, but it is useless for a coin hunter in general..
Old Pro you are very correct in defining the usefulness of any detector relies mainly upon the operator. I will also say that unless you have a ground canceling factor machine, you will end up digging new stock watering dams everywhere. The newer 900 series types are excellent, the older Fisher, while an excellent machine, did not have it and was famous for empty holes due to various other reactions as you menitoned.
Don Jose de La Mancha