GMC 2003 Savana Van Owner's Manual page 40

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 seated in a rear outside seat position,
move the child toward the center of the vehicle. See
Rear Safety Belt Comfort Guides for Children and
Small Adults on page
7-28.
If the child is sitting in the
second row center seat 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 in a
seat that has a lap belt, if your vehicle has one.
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