This possibility is visible when a stone looks like a standard, one-person, tripartite stone, but commemorates more than one person in both its text and its tympanum design.
In many cases, 18th-century gravestones have one lunette for each person named. Examples:
Copp's Hill, Boston
Sometimes, you find stones that have the shape typical of an individual stone, but multiple soul effigies corresponding to multiple burials:
John & Mary Watson
Roby Green & Child
These make me think that, in some cases, customers may have chosen stones that had already been hewn into the standard shape, but were not yet embellished.