GMC Sierra Denali Classic 2007 Owner's Manual page 39

CAUTION:
{
Never do this.
Here two children are wearing the same
belt. The belt can not 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 seat next to a
window, move the child toward the center of
the vehicle. Also see Rear Safety Belt
Comfort Guides on page 35. If the child is
sitting in the center rear seat passenger
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 the belts provide.
39