The problem is the last one standing is often the last one around. My wife's great great grandparents were William and Elizabeth Jones who lived near Chattanooga, TN.
Elizabeth died May 5, 1913 and a nice double marker was put on her grave at the Welsh Rogers Cemetery near Sale Creek, TN.
When William died on October 24, 1918 most of his children had moved away to Texas. He was buried in the same plot but his name was never engraved on the marker.
I discovered the omission recently when a volunteer from Findagrave.com went out to take photos for me.