Multiple Keys for an Encrypted Secret
Here is one possibility, which answers the scenario you asked about, but feels too fragile for real use.

We'll have a trusted dealer, which selects an RSA modulus N (with secret factorization), a large exponent $e$, and two decryption exponents $d_1 = e^{-1} \bmod \lambda(N) + k_1 \lambda(N)$ and $d_2 = e^{-1} \bmod \lambda(N) + k_2 \lambda(N)$, for distinct integers $k_1, k_2$ (and $\lambda(N) = \text{lcm}(p-1, q-1)$)

Then, he encrypts the secret using RSA and exponent $e$; he then hands $d_1$ and $N$ to Alice, and $d_2$ and $N$ to Bob; they can both use these to decrypt the secret using RSA.

Just given $d_1$ or $d_2$ (and $N$), neither of them can recover the other; hence you have the traitor-tracing properties you're looking for.

Of course, if anyone learns two of $e, d_1, d_2$, they can factor (and so can derive their own $d$ values), hence the fragility of the system I mentioned.