complexity theory – Hardness of a problem which is the sum of two NP-Hard problems

Consider the problem of computing an exponential sum over a certain function $g(x)=f(x)+h(x)$, that is computing

$$sum_{x}g(x)=sum_{x}f(x)+sum_{x}h(x)$$

now if we know that $sum_{x}f(x)$ and $sum_{x}h(x)$ are two NP-Hard problems, what can we say about the hardness of $sum_{x}g(x)$?