GMC 2005 Sierra Denali Owner's Manual page 34

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.
1-28
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 the belts provide.
If the child is sitting in a rear seat outside position,
see Rear Safety Belt Comfort Guides for Children
and Small Adults on page 1-24.
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.