A few Trilliums had this option. I think it was mostly (all?) the 4500 model that had a bathroom in front (so there were no other sleeping places except the rear dinette made up into a bed, unless you got the bunk option).
Even so, from what I have seen, few people took this option. It seems rare.
When the bunk was folded down (to use), it partially covered the side and rear windows
, and when it was stowed, it still covered over part of the dinette area (making a lower ceiling). I'm not sure if people on the lower bunk could still sit up in bed then - probably shorter people could; not sure about taller ones.
In the case of the Trillium
, the front (open) edge of the deployed bunk spanned the full width of the trailer and special metal "arms" came down on the sidewalls at the forward and rear edges of the side windows
and then a bar type thing between them caught the bunk. The bunk was wider than a single, but not a true double. I don't remember for sure, but I would guess it was about 38-40" wide.
I have seen a few mods that added a bunk over the "main" bed - maybe one of them was in Chester Taje's Boler
I think you'd have to really want/need one, as it would encroach on the "best" end of the trailer; but then, part of the fun is looking around the camper and wondering "What if I..."
Edited to add photos of the bunk stowed and deployed in a Trillium
4500 (which is a bit wider and longer than a U-haul/Burro 13). The "bunk up (stowed)" one was pretty dark, so I lightened it. Quality is not great, but at least I hope you can see the gist.