I agree Redsy.
210.4(B) requires simultaneous disconnection of the ungrounded conductors that supply stuff on a common yoke. A listed handle tie would take care of that.
I see frequent examples of a 15 A and a 20 A circuit sharing a common 12 gauge neutral in residential systems based on EMT or Flex wiring methods. In this area, 7-county Minneapolis / St. Paul, the age of installation is most commonly 1950 thru the mid-70's. In 1975, the local "all metal" requirement was relaxed to permit NM.
Perhaps the most common configuration was running a general lighting circuit with the laundry circuit.
The slight economic incentive to save on material used is still available in NM if one uses a 12/3 NM homerun with one of the ungrounded 12s on a 15 A breaker, and the rest of the 15 A circuit beyond the 12/3 on 14/2 NM.