Remodel gone bad

[JohnKeating]

An Island with three closets was removed. Two light switches, thermostat, doorbell transformer and 1 duplex were in the walls.
Pretty basic but the red flag that popped up was 2 circuits on the same phase feeding the 120V.
I guessed that the install was accidentally tied by the previous "worker" since one of the feeds was NM and the other was old style braided cover romex.
Now after a few weeks the neut is gone in several duplex and lighting fixtures. Trouble shooting last night I found a bad GFCI, two toasty looking duplexes, and think this may be a can of worms.
If I rejoin the two circuits there is a good chance of overloading the neutral(s) somewhere.
Thanks for any wildcards

 

[Little Bill]

Are you saying that a MWBC was attempted but they put both hots on the same leg?
Or are you saying that one circuit was double fed?

 

[JohnKeating]

Are you saying that a MWBC was attempted but they put both hots on the same leg?
Or are you saying that one circuit was double fed?

Not sure what a MWBC is but, what I found when the sheetrock was removed is:
Some old, presumably original(1967) braided romex cable, was joined to a newer NM type romex cable in a single gang box with a duplex receptacle.
I moved the switches, thermostat wiring, bell transformer from the studs to their new location. I then split the two circuits and powered the lights, and loads from them. Due to the shortness of the wiring, a single gang box was placed in the crawlspace for one splice, and a two gang was place in a wall about 9' from the floor. Both splices are both accessible.

 

[JohnKeating]

Ok multi wire branch circuit.
No two single cables, same phase, both hot. Spliced.

 

[Little Bill]

Ok multi wire branch circuit.
No two single cables, same phase, both hot. Spliced.

Then just locate the two breakers and disconnect one of them. Or tie them together and pigtail them to one breaker and neutral bar.