If you are really proficient on this method, you can reset the camera to view a second adjacent panorama and overlap the frame slightly to come up the possibility of stitching the 2 negs/transparencies from one sheet to make a 2X9 inch panorama.
One other advantage given relatively same exposures, you can scan the two panoramas as one 4X5 and then separate them digitally.
Regarding the Chinese panorama backs. I've heard varying reports, some very good. However, the pricing seems a bit odd until you make the distinction that there are panoramic backs that ONLY do 6X12, and then there are backs that are multiformat, ie. 6X6, 6X7, 6X9 and 6X12 using masks and multiple red windows.
The dedicated 6X12 format backs are less expensive.