Let's suppose we are using the exponential ElGamal as a public-key encryption scheme, so that we encrypt $g^m$ instead of $m$, for some generator $g$. Let $x$ be the private key, and $h=g^x$ be the public key.
We have two parties, and each one of them has a ciphertext encrypted with the same public key: $(R_1,S_1)=(g^{r_1}, g^{m_1} h^{r_1})$ and $(R_2,S_2)=(g^{r_2}, g^{m_2} h^{r_2})$, respectively.
These two parties then gather and perform the homomorphic sum of their ciphertexts by computing the product of their ciphertexts: $(R_3,S_3) = (R_1,S_1) \cdot (R_2, S_2) = (g^{r_1+r_2}, g^{m_1+m_2} h^{r_1+r_2})$.
Is there any way they can prove in zero-knowledge that the sum is correct? That is to say, that $(R_3,S_3)$ is the correct encryption of $m_1+m_2$ without revealing anything else about the addends ($m_1$ and $m_2$ should be kept secret) nor the (plaintext) value $m_1+m_2$?