600 amp breaker at switchgear. parrallel 500's going to switch fused at 400amp. tap box was just installed just before switch. Parrallel 3/0 now supply the switch. a tap of 3/0 will supply trans. Its the 3/0 to the trans that we have only taped to one set of 500's.
Assuming 500s in separate conduits, 380 x 2 = 720, ok at 600a breaker/load.
If 500s in single conduit, 380 x 2 = 720, 720 x .8 = 576, ok at 600 a OCP, 576a load.
Tap to the switch from
both 500s with parallel 3/0s to the switch must comply with 240.21 (B).
Tap to the transformer from
both 500s need to terminate in the proper OCP, or comply with 240.21(B)(3).
But as Chris pointed out, both 500s need to be terminated in the same manner.