Just for your Info--- It looks like a good idea
Carlon has introduced a new box for use in shallow walls (furred out block walls 1/1/2” thick) . It looks like a shallow 2 gang retrofit box but the second half is about ¾” shallower than the rest of the box. This part slips in behind the sheetrock. Here is a picture of the box without the cover on the section that goes behind the sheetrock.
Interestingly enough an electrical contractor brought that box into the local electrical supply this morning. I said where’s the cover and he said it doesn’t come with a cover. I said it isn’t legal without a cover, it must have one. He said the EI said as long as it said UL it was Okay. I said no it ain’t. Fortunately, the supply house had internet access. I found the PDF file and showed him the box with the cover. I bet the Home Depot crew, where he bought the box, threw the covers out not knowing what they were.
Here is the link--- go to page 6 (8 of 22). The part # is B117RSW