Burying in the same grave space

Question

What halachic rules (or minhagim) apply to burying more than one person in a single grave space, in other words one coffin on top of another? Is it permissible for husband and wife, or father/son or mother/daughter, or same gender siblings? Must the two individuals be the same gender? Must they be related to each other? Is there a minimum vertical distance required between the coffins? Assuming the deaths are separated by time does the first burial need to be modified (the coffin deeper than normal in the ground) to enable the second above it at a later date? Does it make a difference if the reason is lack of space, or personal preference where there is no lack of space?