Changing from a y-pipe/cat single exhaust to duals can be a problem if you have emissions inspection. It's a violation of federal EPA "regulation" (read "laws we didn't get any voice in") to change the number of cats from the factory configuration.
That means that it's technically illegal to remove a cat or to add one.
If you live in an area where there's no emission testing, it's not a problem. But if you have to pass a visual inspection....well, it could be a real big problem.
With that said, if I were to live in an area without smog testing, I'd throw headers on the truck (I'm assuming this is on the 76 or so truck with the V8, right?), then just have custom tailpipes made up with mufflers and no cats. I know, this is totally outside the letter and spirit of the law, but I'd do it anyway. It's what I did with my 80 El Camino when I dropped a 406 "twin-four" into it to replace the ailing V6. That's been about 7 years ago and I have to get it smog tested every year, but they don't do a visual on that year vehicle. I just have to get it to run clean enough to pass the sniffer test. That's enough trouble in itself.
Back to your original question about using the current cat and muffler: it's almost impossible to say, because I have no idea what you have on there now. Catalytic converters are built to handle a certain amount of exhaust gas and depend on temperature build-up to operate properly. So if you have 2 stock cats instead of 1, then each is going to get approximately half of its intended exhaust gas flow. They may never get fully up to operating temperature and not do much good as far as decreasing emissions.
I also have no idea what muffler is already on there, but it's pretty unusual to go from a single to a dual exhaust without installing new mufflers. I don't know what you're looking for in the way of exhaust tone so I can't recommend anything. I'm kind of an old-skool guy and I like the sound of glasspacks. But not everybody does. So the end result as far as sound will have to be up to you.
I'm sure this has caused more confusion than clarity, but I think it needed to be stated. You may get some better answers about the specifics of your area and the laws you have to conform to by seeking out a forum that deals more with later-model trucks (which are still considered by many to be "old trucks.")
Good luck,
