It’s a Dr. Zoidberg balaclava! OMG…if I weren’t an almost 40-year-old man I’d squee.
Go ahead and squee, we don’t want you to hurt anything. 
I teach jacquard/double-knitting. I couldn’t IMAGINE doing a piece the size of that poncho with the technique. What a HUGE investment in time, not to mention materials. Plus it would be incredibly warm; you’d only want to wear it on your visits to Antarctica. In the winter. Outside.
Also, for me the entire point of jacquard is doing color patterns (although I also use the technique to make socks, which I make in a double layer, then close the toe and open the “tube”). So making such a huge project with one color would not only be very, very difficult to keep track of which side your respective strands are on, it would also be suicidally monotonous. And doing it in the single strand knit/slip technique–merely contemplating it makes me want to cut myself.
Also, keeping the strands perfectly separate will give you two sides that are not anchored together. These will shift over time. Guaranteed. The twist in the yarn creates an insidious creep that becomes more and more pronounced over time; the moreso the larger the piece.
If you honestly have nothing else to do for the next six months and want to try to do this in two layers, do it with a color pattern: every vertical color change will tack the layers together (horizontal stripes do not tack; you will still get creep). You’ll have a firm, solid fabric of double thickness, rather than a huge tube slip sliding around on itself.
Personally for something this size I’d sooner do it either in aggressively blocked stockinette, or some non curling stitch like seed or checker.
Good luck. Report back. If we don’t hear from you for the next several months we’ll know what you decided to do.
I love the phrase ‘aggressively blocked’ - it makes knitting sound so hardcore.
I’m inclined to agree with you lissener - the garment I was looking at was NOT a tube which makes me think the single strand knit/slip technique was not used.
I think I’ll go price out yarn - bamboo does tend to be pricy around these parts, and depending this thing could wind up costing more than the $180 the gal was asking in the factory which would make me feel dumb.
Also, I’m due in about 3 months and have a number of other things I’m finishing for Jr. first including an entire knitted sock monkey family, as well as an absurd striped baby romper (hey - what’s the point in having a kid if you can’t dress them in goofy outfits?) and I’d actually like to have my nursing shawl ready while I’m still nursing.
I might add some stitch work to make things interesting, or more likely just do it plain - I could make a big stocking stitch rectangle on a circular needle with my eyes closed (literally) so that may make life easier and save the fiddley stuff for baby.
You can do all the edging in an inch or so of jacquard. That will avoid the curl, and give the edges some weight to keep the poncho a little easier to manage in a wind. That way you can do a patterned edge around an solid center, which might be kinda cool.
You could also do it like a circular shawl, except start it with the head hole already in place in the center so you not only have a ready-made poncho, but you don’t have to do the fiddly dpn stuff. It would also make the stockinette much easier, as you’d only have to knit, instead of adding in the purl rows.
The only actual work involved (other than the knitting) is figuring out your increases.
If it was double knit, maybe it was done on a large Knitting board? It’s not really knitting, it’s more like wrapping the yarn on the spokes and using a tool to move the lower thread over the upper thread. Like a knitting spool, but long and flat.
This is an interesting idea; however, I kind of liked the simple oblong of this particular item - I have made circular shawls before but this one had a different sort of look.
I may edge it in jacquard as lissener suggested, or I might edge it with a lace stitch pattern, or I may just let it curl up or I may aggressively block it just 'cus I like that way that sounds.
Thanks everyone for the suggestions - now I’m keen to go and start yarn shopping. 
You can make a rectangular shawl in a circular fashion; just keep all your increases at the same point. But the shawl you describe sounds more interesting.