In iguana, atomic cross chain swaps are supported, but a coin has to support at a minimum a 2of2 multisig to an arbitrarily specified pair of secp256 pubkeys.
the above shows how to sign a transaction, but not specifically for a 2of2 multisig. Iguana has C library for secp256 already, so the actual signing code is already done. dealing with creating a 2of2 multisig account, serializing it properly for signing, submitting to the p2p network are the parts that are needed.
I realize this "spec" is not 100% precise so the first part would be to come up with a clear interface. please respond if you are able to do this