Secp256k1 is a curve, not a scheme - you can build different schemes over it that have different private/public key relationships.
If you do plain ElGamal on secp256k1 for example, $Q_A = D_A . P$ for a public base point $P$ so you get the usual key-sharing properties: if my keypair is $(D_A, Q_A)$ and yours is $(D_B, Q_B)$ then anyone can compute $Q = Q_A + Q_B$ to produce a public key whose secret key is $D = D_A + D_B$. If they can encrypt a message under $Q$, you and me have to cooperate to decrypt it as the secret key is effectively $2:2$ secret-shared.
In summary: if the function mapping secret to public keys is linear (which it often is) then one can transform linear relationships in the way you asked for.