GMC 2003 Yukon Owner's Manual page 51

{
CAUTION:
Never do this.
Here two children are wearing the same belt.
The belt can't properly spread the impact
forces. In a crash, the two children can be
crushed together and seriously injured. A belt
must be used by only one person at a time.
Q:
What if a child is wearing a lap-shoulder belt,
but the child is so small that the shoulder belt
is very close to the child's face or neck?
A:
If the child is sitting in a rear seat outside position,
move the child toward the center of the vehicle.
See Rear Safety Belt Comfort Guides for Children
and Small Adults on page 1-41 . If the child is
sitting in the second row center position, move the
child toward the safety belt buckle. In either
case, be sure that the shoulder belt still is on the
child's shoulder, so that in a crash the child's
upper body would have the restraint that belts
provide. If the child is so small that the shoulder
belt is still very close to the child's face or
neck, you might want to place the child a seat that
has a lap belt, if your vehicle has one.
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