LeGrand (Pass & Seymour) make a multi-control single pole switch with a split tab TM8113WCC, which allows for the control of two independent circuits on a single yoke (one box). Code question: is it permissible to run a mains voltage (120V) circuit on one side of this device and a low-voltage circuit on the other?
This is OK provided that the wires to the fan control switch are treated as 120VAC wires as per 725.46:
Class 1 Circuit Wiring Methods. Class I circuits shall be installed in accordance with Part I of Article 300 and with the wiring methods from the appropriate articles in Chapter 3.
(i.e. run using NM and not communications-type cable/wire) and as per 725.48(B)(1):
(B) Class 1 Circuits with Power-Supply Circuits. Class 1 circuits shall be permitted to be installed with power-supply conductors as specified in 725.48(B)(1) through (B)(4).
(1) In a Cable, Enclosure, or Raceway. Class 1 circuits and power-supply circuits shall be permitted to occupy the same cable, enclosure, or raceway only where the equipment powered is functionally associated.
since the control wires are with the fan's power wires, and we can safely treat them as a Class 1 control circuit using the provisions of 725.130, Exception 2:
Exception No.2: Class 2 and Class 3 circuits shall be permitted to be reclassified and installed as Class 1 circuits if the Class 2 and Class 3 markings required in 725.124 are eliminated and the entire circuit is installed using the wiring methods and materials in accordance with Part II, Class 1 circuits.