If there exists a multi-key CKKS scheme in addition to the threshold CKKS scheme?"
Yes, please see the following papers for details:
- Efficient Multi-Key Homomorphic Encryption with Packed Ciphertexts with Application to Oblivious Neural Network Inference
- Asymptotically Faster Multi-Key Homomorphic Encryption from Homomorphic Gadget Decomposition
Also see Implementations of multi-key FHE schemes for more information.