GMC Yukon Denalid Owner's Manual page 62

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.
62
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 rear seat outside
position, move the child toward the center
of the vehicle. If the child is sitting in the
second row center 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
that belts provide. See Rear Safety Belt
Comfort Guides on page 56. 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 rear seat that has a lap
belt, if your vehicle has one.