That is true, you are doubling the fuel area. But the flow isn't exactly double for each carb.
Holley most definetly does not recommend using two 390s on a V8. The 390 is recommended for single carb applications on small dispalcement V8s and performance 6 cylinder engines with aftermarket intakes.
If you ever get the chance, take the powervalve gasket out of one carb on a dual carb setup. Let the engine run for a little bit. There will be dew and ice forming. It is because of the over rich condition. The excess fuel does evaporate inside the tunnel ram plenum.
Running lean will not cool things down. Think about it.