So far, I can only find one
"POSSIBLE" NEC issue - 210.4(A):
(A) General. Branch circuits recognized by this article
shall be permitted as multiwire circuits. A multiwire circuit
shall be permitted to be considered as multiple circuits. All
conductors shall originate from the same panelboard or
similar distribution equipment.
In a _Technical_sense... I do not see any performance issues, or "Smoke-Loss problems" with the current installation.
Albeit, I would have chosen to bring the Grounded Neutral Conductors of the relocated Circuits, to the newly installed Panelboard. This would be done, so as to "Keep The Circuitry Together"
Similar to what others have mentioned, if an L-L 2 Wire Branch Circuit is available, I would also choose to relocate that Circuit to the new Panelboard, and use its original 2 pole spaces for the new Feeder Device.
This setup appears to be an existing 200 Amp, 120/240V 1 Phase 3 Wire Service Panelboard (Meter Section + Main Service Disconnect & MBJ, with 200 Amp Panelboard combined), and the OP has installed a new 100 Amp, 120/240V 1 Phase 3 Wire Panelboard directly next to it - and fed the new Panelboard with a new 100 Amp Circuit via 1-1/2" PVC Nipple. New Panelboard Feeder is derived from 100/2 MCCB installed in existing Service Panelboard at positions of the existing, relocated Branch Circuit.
So much for Mr. Obvious, huh???
Per the new Grounded Feeder Conductor, I am also curious how the OP terminated the #2 (or #4?) Neutral Feeder to the Existing Service Panelboard's Neutral Bus Kit, due to the "Can-O'-Worms effect" surrounding the Neutral Bus (packed full and hard and dangerous to get into).
Back in the day when I actually worked hard in this trade (read: when I worked in the field), I had dealt with a few Services which had a similar "Dangerous" placement of the Neutral Bus Kit, and I needed to remove and/or add Conductors to it.
Turning off the Main Disconnect was good on some, but others were just too dangerous (or no Main Disconnect).
In these cases, we would:
* Pull the kWH Meter (when the sealing rings were the thin Aluminum ones),
or,
* Cut the Service Entrance at the Drip Loop (where the Meter sealing Ring was the "Tamper-Proof type).
I can recall some Services with "High Pucker Values" to be ITE / Bulldog "Pushmatic", Zinsco type Q, Square D type XO (and some earlier QO Panelboards - the "Thin Ones"), some Crouse-Hinds types, Challenger circa 1980's, and of course FPE's "Stab-Lok Panellettes"
It's been so long, I can hardly remember what a Residential Service Panelboard looks like!!!
Adding my $0.02
Scott