GMC 2006 Envoy 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 1-28. 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.
1-33