The reason why their is 2 sets of 3/0 AWG to 200A fused disco because the 2 sets of 3/0 is tapped from wire in trough that has 2 sets of 500 kcmil.
There are other wire tapped besides 2 sets of 3/0 to other disconnects from trough.
If one uses 1 set of 3/0 then would not it cause imbalance in 2 sets of 500kcmil cable in trough?
Indeed, you do not want to introduce an imbalance by only tapping one of the feeder sets in parallel, as they depend on the symmetry for their theory of operation. The code wording doesn't exactly state this, but the need for symmetry is strongly implied from the general rules for parallel conductors.
To resolve this situation, make your tap with a Polaris block or equivalent, that has 5 terminals. Bring the two sets of 500 kcmil together, initiate the 200A tap with 3/0 Cu, and use the two remaining terminals for the two sets of 500 kcmil that travel to the load. Another solution is to use a 400A disconnect, with 200A fuse reducers, so you have two tap destination terminals by design. Or anything else that allows the two sets in parallel, but this is rare for 200A & less equipment. Usually you are looking at 300A to 600A, where the manufacturer gives you two terminals by design.
If it were anything other than a tap, like a way to mitigate voltage drop on a feeder, you'd be able to use a 3-terminal Polaris block to step 2x 3/0 down to 1x 3/0, to terminate on the disconnect. But tap rules require that your conductors terminate in the overcurrent device, whether it is a breaker or fused disconnect. The tap rules don't allow you to terminate in anything other than an overcurrent device. Only the obscure 100 ft tap rule specifically disallows splices, but I wouldn't recommend relying on a 2-to-1 set reduction being considered "just a splice".
