A double tapped circuit breaker (Two wires entering the same breaker from two separate circuits) is one of the most common electrical defects that home inspectors find when checking the main electric panel. The problem with putting two wires in a circuit breaker designed to hold one is that the wires could become loose, even if they feel very tight. Loose wires can lead to overheating, arcing, and possible fire.
Some breakers and connections are approved for double tapping with a few restrictions i.e. the wire size of both wire entering the breaker should be of the same gauge.
Even if the breaker is approved for double tapping, a concern would be a possible Code violation, in how many outlets are on the circuit. If one of the circuits was for a kitchen fridge then likely the double tap would be illegal, unless it was during a time when other items like the hood fan were allowed on that circuit in the 80s. If it were say a number of bedrooms with two feeds coming in and the total was 12 outlets or less the Code would not be violated. It all depends on the circuit, the breaker type, the total number of outlets and the method used.
1. If the circuit was properly loaded but the breaker was not approved for two taps, then a simple marrette connection (providing there is space in the panel) would clear up the issue.
2. Install a new breaker that is approved for use with the panel & designed to hold two wires.
3. Install a new breaker if there is room or install a slim line breaker & split the two wires off to separate breakers.
When this condition is observed it is always recommended that the breaker be checked for safety by a licensed electrician.