GMC 2005 Envoy Owner's Manual page 38

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.
1-32
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 for
Children and Small Adults on page 1-27. 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.