The FAA just published a clarification that might be important to some
soaring operations. It clarifies when two passengers can share a seat belt.

"This action clarifies prior interpretations of FAA's seat belt and
seating requirements. These prior interpretations state that the shared
use of a single restraint may be permissible. This clarification states
that the use of a seat belt and/or seat by more than one occupant is
permitted only if the seat usage conforms to the limitations contained
in the approved portion of the Airplane Flight Manual (AFM). In
addition, before multiple occupants use the same seat and/or seat belt,
if the pertinent information is available, the pilot in command (PIC)
must also check whether: The seat belt is approved and rated for such
use; and the structural strength requirements for the seat are not
exceeded. This clarification also emphasizes that, because it is safer
for each individual person to have his or her own seat and seat belt,
whenever possible, each person onboard an aircraft should voluntarily be
seated in a separate seat and be restrained by a separate seat belt."

Full Text: http://tinyurl.com/d7s66un