Cadillac 2005 XLR Owner's Manual page 26

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.
1-20
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 that belts provide.