The only aircraft I know of where it is intended that two persons share a seat belt is th Piper Supercriuser. The fact that this was approved for that aircraft does not make it a good idea. Most aircraft have a specified limitation of the number of occupants, which with the foregoing exception, will equal the number of seatbelts. They are also insured for a maximum number of passengers. Two good reasons to not exceed the passnger number regardless of weight.
Let along legality, for the sake of safety I would simply never do this.
In the case where Airlines permit an infant to occupy the same seat as a parent, a second seatbelt is provided to independantly secure the infant to the partent's seatbelt without being within it. A parent just holding the infant is a bad idea. Let's say the infant weighs 20 pounds, and were in the recent Turkish airlines crash in Amsterdam. That plane stopped at somewhere around 20G forward deleration. During that time, the 20 pound infant now imposed a 400 pound force to be restrained. Could the parent hold that load in the best of times, let alone during a crash?
Pilot DAR