If it is absolutely illegal to remove the cat, can I add another cat to make it a true dual unit,QUOTE]
In California, the answer is "no". The law says that if the vehicle came from the factory with a single cat then it must stay that way. The only way you can add another cat is if the vehicle came factory equipped/optioned with 2 cats. Which few vehicles in the 70's did.
The "emissions legal" option? Run 2 1/4 from the manifolds to the head pipe, then increase to 3". Run it into a 3-4" performance cat...tons of aftermarket companies make these. And then run a 3" all the way out the back. This is similar to what some aftermarket exhaust companies were offering for 93-97 LT1 Camaros and Firebirds when they first came out.
Or you can go with your 2nd option to upgrade the cat and split the tailpipes behind the muffler.
California doesn't require emissions testing on 1975 vehicles and earlier.