A Tandem Suspension is a suspension in which one person is suspended by hooks from another person. This suspension was originally named by (and first done in the West by) Allen Falkner, who first did it with Oliver from TSD. That is, one person is suspended normally, and then a second set of hooks is attached below them. The ropes from these hooks then suspend the second person (ie. it's not two people suspending side-by-side).
Many configurations are possible of course, and blood bowls can be used to stop blood from running between the ropes from one suspendee to the next.