Double-decker burial plots are very popular at San Gorgonio Memorial Park in California.
JoAnne Gosen, assistant manager of the public cemetery, estimated at least 75 percent of the couples who reserve graves buy the double option, with the casket of the first to die buried deeply and the next on top. About the only restriction is that they share one grave marker.
Mostly, the double-grave occupants are married couples, but some are parents and children and even pairs of cousins. Gosen thinks there are probably friends who are not related at all buried together. [Link]