Update Looking at the source, it seems like the initial key exchange keys are sent as a k_(pub,x) || sig tuple where x is 1 for the sender and 2 for the receiv

Update Looking at the source, it seems like the initial key exchange keys are sent as a k_(pub,x) || sig tuple where x is 1 for the sender and 2 for the receiver. Similarly, it looks like the shared keys are derived from subslices of k_(pub,x) and k_(priv,3-x). Is there a particular reason there isn't a single read/write symmetric key that's derived from the entirety of the public and private keys?