Encryption using two keys
Yes, it is possible with a symmetric system.

One way would be if we had a public prime $p$, and secret keys $k_1, k_2$ which are values relatively prime to $p-1$.

And, we define $E_k(M) = M^k \bmod p$; decryption then is $D_k(M) = M^{k^{-1} \bmod p-1} \bmod p$ (the value $k^{-1} \bmod p-1$ is easy to compute, given $k$).

And, for $K_{12} = K_1 \cdot K_2 \bmod p-1$, we have $E_{K_{12}}(M) = E_{K_1}(E_{K_2}(M))$