Bert Coules wrote:
I've no idea if this in an approved method (and rather suspect that it
isn't) but I've mounted several sets of backboxes-in-a-row by first
screwing them to a wooden backplate and then putting the whole
assembly into a slightly oversized hole partially pre-filled with
plaster/cement/mortar/what have you.

The beauty of this approach is that it ensures that the boxes, and
therefore the front plates, are all perfectly in line and evenly
spaced - something I could never achieve when I used to do it one box
at a time.

Do you want a job?

The only difference is that I mount the wooden board to the wall and then
fit the back boxes.

--
Adam