I'm trying to create an encrypted partition on a server which requires two passphrases from a pool of 4 to unlock - all keys can work together. The idea being the disk is usually unmounted unless we want to gain access, then two people can use their keys to unlock it - it must be a combination of any two though otherwise it defeats the object. Additionally, these two people can be at different locations and accessing the box via SSH.

Is this possible?

In short "no"... primarily because encrypted partitions use symmetric encryption which uses a single key (i.e. one key to decrypt and encrypt) and you can't have two keys at once....

However, you can treat the key to the encrypted partition as a "session key" (similar in concept to PGP) in which you encrypt it using the RSA keys of your intended "key holders". What you describe is a bit more tricky though because what I described allows any one of the multiple keys to decrypt the session key. In your case you would need to encrypt the session key multiple times with said keys.

For example, one person decrypts the first stage, and the second person takes the output from the first person and uses their RSA key to decrypt the session key. This can get kind of messy though.

What is the problem you're trying to solve? It sounds pretty edge case and unique.

Right, this sounds like it's getting far messier than I want. It's a proof of concept more than anything else - it's basically secure storage of a few files on a remote server which won't be accessed often. The reason for requiring two keys is for security - we don't want anyone to gain access to the files by themselves, for security reasons there needs to be two people involved. Obviously this doesn't rule out collusion, but it makes it that much harder to gain access to the files if you want to do something nefarious/malicious.

Is there any other way you can think of which would achieve roughly the same purpose?