GMC 2003 GMT560 T-Series Owner's Manual page 26

{
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.
1-20
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:
Move the child toward the center of the vehicle,
but 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.