Sven,
NEC (2002 edition) 110.14(A) has a reference to more than one wire per terminal:
Terminals for more than one conductor and terminals for aluminum shall be so identified.
110.3(B) states:
Installation and Use. Listed or labeled equipment shall be installed and used in accordance with any instructions included in the listing or labeling.
Put those two together, and I reckon that's covered the issue. Maybe one of our NEC experts can confirm or add something to this.
