The only V-8s that originally had dual exhaust were pre 86 models from outside the US. Those grey market cars had cast-iron manifolds that exited on either side of the trans. and there was plenty of room to fabricate cats. into the system.As far as being legal I doubt most states would allow modification fron the original configuration. If it only has to pass a sniff test then I'm sure a system could be made that would pass.
Starting with the 86 models the left manifold crossed over the trans except the SLs which crossed under the bellhsg. and merged into one large cat setup which became dual after the cat.
The best way would be to find AMG headers that were made by another co. Sebring? maybe and of course were big bucks. One more thought, the port shape on the 420-560 motors was offset and would match poorly with a 380-500 manifold. Good Luck!