There is no setup for the 63 and above to run dual carbs. 62 and below have an take manifold so you can run dual carbs.

Running dual carbs is more of a status statement with Cadillac, than it is with performance.

Not getting into the pros vs cons of multi-carb set-ups, but this did trigger some thought I had on this way back on multi carbs on the 63/64 Cadillacs.  
In the 1966, 67 and 68 Master Parts Book parts group 3.265 the intake manifold for single carb applications have a different intake manifold from 59 to 62 (part # 351 2079) vs the 63 to 66 intake (part # 363 2870), yet in the 1969 Master Parts Book it lists 59 to 66 as the same intake part # (363 2870). I know the cylinder heads have the same mounting areas since the 1958 to 1963 heads were the same, however, I also know the engine block physically and dimensionally changed between 1962 and 1963 with the core block reducing length and height. I don’t know if the core block change altered the relative position of the intake on the heads and if it would be a straight bold up, but it seems going with the 69 Master parts book you can use the earlier multi-carb intake manifolds on our 63/64 Cadillacs.

Of course you would have a lot more work cut out for you in terms of accelerator linkage, fuel lines, air filter(s), etc.  If you were after all out performance the EFI and fuel injections options should produce the best power and performance gains, however, for Old-School Cool it would be neat to see a dual or triple carb setup on a 63 or 64. If someone knows for sure I would like to know so I can update my notes.

I have a pre 62 dual carb intake. I'll have to see if it fits the 63/64 engine assembly.