Not sure if I understand. If your illustration with the 4/0 was changed with 350 to the left and right. A polaris lug for the350 to #3's. Is this right? At the 1200 fused main would be a 4 barrel lug for (4)350's.
In my illustration you would have (4) 350's going to the left as well as 4 to the right if there were more taps to the right.
I have a feeling you may be thinking (4) 350's landing in the 1200 amp disconnect with 2 going left and 2 going right. If that is the case you do not have a 1200 amp parallel conductor you have 2 taps that each supply additional taps.
To be able to call them parallel conductors they must be connected together at each end effectively making them one.
Like I said it may be difficult to find a splicing device that will accept 4 - 350's and 10 - 3AWG's. If you wanted separate feeders to left and right you need to come up with 8 barrel lugs for the 1200 amp disconnect, run 4 - 350's left, 4- 350's right, and then come up with splicing devices that will accomodate 4 - 350's for both sides and 6 taps on left, 4 taps right.
If you were starting from scratch a meter center would be the way to go. Good luck trying to straighten this out.