Is Another Grounded Conductor (Neutral) Required When Adding Another Circuit to a Switch Box?

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J&J

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Retired military, aspiring engine builder and handyman
I didn't see a post that covered this, but it must be out there. Apologies in advance...

My municipality operates under the 2018 NEC. I think I'm covered by 404.2(C)(5). For what it's worth this is "old work".

I'm starting with an existing 1-gang box for a switched circuit that controls lighting. It already has a grounded (neutral) in the box for that circuit.

I'm making this a 2-gang box and adding a separate switched circuit (separate home run) to control new half-switched duplex outlets.

I believe I can add the separate circuit without it's own neutral in the 2-gang switch box because as mentioned, the other circuit already has one in that switch box.

Detail:

I want to use a single 12/2 wire in a switch leg config between the new j-box and the new switch (in the converted 2-gang box). Then 12/3 between the new j-box and new half-switched outlets.

Electrical flow: A new hot would run into the new j-box from a new sub-panel (via 12/2). Then a new 12/2 from the new j-box to the new switch in the converted 2-gang box (black hot in, white switched out, no neutral from that new circuit). Then the white (switched) conductor would connect to a new switched (red) conductor in the new j-box, then to the new outlets (via 12/3) as black (always hot) in, red (switched) in, white neutral out.

Alternatively, I could run 12/3 from the new j-box into the converted 2-gang switch box so that new circuit has it's own neutral in there. But I couldn't connect to anything. In my opinion, the new white/neutral from the new 12/3 is redundant since the existing 12/2 neutral is already in that 2-gang box. I hope I'm explaining his well enough.

I'm pretty certain this is legit, but please shoot it full of holes if/where you see them.
 
Different neutrals from different circuits can occupy the same box but they should not be electrically connected in most case. A multiwire circuit can share a neutral but two separate circuits should not. The reason for the neutrals in the sw box is for occupancy sensors etc. If the circuit that someone installs an occupancy sensor on does not have a neutral for the circuit the sensor is controlling, I don't think you can use another neutral that happens to be in the box.
 
J&J, something specific in your post suggests you're not a qualified professional per the site owner's rules.
 
Please try diychatroom.com

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