I have sewn a sanghati by this method. Personally, I would not recommend it. You end up with something that is a true marvel of sewing ingenuity. It’s quite awe inspiring. The method ensures that there are no visible stitches showing. However, other than bragging rights, I seen no benefit whatsoever.
I was living at a monastery where there were constantly new monks coming through the ranks, so the method was constantly in use by at least one monk. As I understood, the technique was passed on person to person, with those who had not done it in the last year quickly forgetting exactly how to do it. On top of that, it requires that both layers of cloth have exactly the same pattern drawn on, otherwise each section that was off would cause poofyness when completed. It also requires making internal cuts in the fabric that are easily done incorrectly. A wrong snip could botch things. And overall the method was so convoluted that making mistakes were inevitable, although unless they involved cuts, could be easily undone. Just demoralizing.
I’m happy to pm you the pattern and instructions I have, but I can say that even having done it myself once, the instructions make little sense to me and I’m sure I could not use them to make a robe easily. But you may have more luck!
This all kind of goes to the heart of my argument, namely, could the monks have really been using methods that required such a high level of skill? With this type of inside-out sanghati, it seems more like an elite practice that mostly serve to show group identity and not further spiritual practice. And if I am remembering correctly, I don’ think the method is well suited to rag robe construction. Personally I think the method I outline above for making a single robe is perfectly suited to making a double layer robe.