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.