I don't understand how fuel could leak from the manifold/base plate gasket ?
It should leak inside the manifold ou outside, but not from inside to outside ?
IMHO, you could face same problem I've got recently. I noticed a fuel leak which really seemed to come from the gasket between manifold and carb base plate (when following the leakage). I really wonder how it could be possible ????
In fact, I discovered that a fuel bowl screw had a stripped thread, and fuel was leaking from the screw head, running under the fuel bowl to the manifold and then going down along the manifold.
I put Helicoils in the 4 screw holes, and solved the problem.
But put a second gasket over the first one should not correct such a problem. And use manifold to carb gaskets is not the way you shoud fix a fuel leak IMHO again...