Am I hearing things?

Is there any other issues I should know for a COMMON NEUTRAL*

Hi Elizabeth,

The first thing I would wonder is if the neutral wiring that is common to the two circuits is rated to be able to conduct 40 amps, which it would have to if and when 20 amps is being drawn from each of the circuits.

Also, I can envision that sort of arrangement being conducive to ground loop issues in some systems, because the unequal currents in the hot and neutral conductors, and their differing distances from the safety ground conductors, would tend to increase hum and noise voltages that are electromagnetically induced in the safety grounds. (I’m assuming that there are safety ground conductors, and that they are physically close to either the hot or neutral conductors)

Jim (Jea48) is more knowledgeable about this kind of thing than I am, though, especially from a code standpoint. We’ll see what he has to say.

Best regards,
-- Al

Elizabeth  one other thing you might look for the two breakers that share the common neutral need to be ganged. 2  20amp breakers with a common neutral will trip like any other 20amp breaker they won't draw anywhere close to 40amps. The load wouldn't be balanced on the neutral which is what would trip the breaker actually breakers they are suppose to be ganged with the hot on seperate phases. 

I don't get the 25amp breaker on a simple branch to a single 20amp receptacle. Those are not common breakers and are mostly used for AC units and water heaters. You said they did some work on your AC are you sure you're not looking at the AC breaker? If not it should be switched to a 20amp breaker.