Two 14/2 source cables connect to one 12/3 cable
This wiring is easier to fix than you think
Since the culprit run of wire was done using THHN in conduit instead of a cable such as NM or AC (BX), this is actually quite readily fixable.  Simply grab a suitable length of 12AWG grey or red/white striped THHN wire, run it through the conduit (this may require removing the existing wires and repulling them), and move the 2nd circuit's neutral over to the new wire at both ends.
(P.S. you only need one new wire as it's legal to have any number of circuits share a single grounding wire; in fact, if your conduit was made of metal, it would be a legal grounding conductor.)
Replace the 20A breakers with 15A ones
Since your circuits have 14 AWG wiring in them, you need to use 15A breakers.  You can only put a 20A breaker on a general receptacle circuit if every piece of wiring on it is 12 AWG or larger.