Multiple private keys
You can use different key pairs for this. You can give both Bob and Cesar two public keys that they can use to encrypt documents with. Those public keys are part of a key pair that is shared with Bob and Cesar - one for Bob and one for Cesar.

Bob or Cesar encrypt the document by first encrypting a two random symmetric keys with the public keys (one each) and using the result of XOR-ing the two random values as another symmetric (data) key. Then either of them needs to encrypt the document with the symmetric data key. To decrypt they both need to decrypt both symmetric keys, combine them and then decrypt the document. They can do this in any order, as long as the two symmetric keys become available at the same time.

This is a fairly basic secret sharing scheme with an asymmetric component.



Undoubtedly there are better schemes that have additional properties. Having some kind of signing or authentication should be high on your wish list as well.