How to partially sign a transaction?

My scenario is I am trying to register my stake pool with funds and a stake addr from a 2nd party. The solution to this would be to sign the registration certificate with the pool keys and then send the partially signed certificate to the 2nd party to sign with the rest of the keys. Once this is done I then need to do the same for the certificate submission transactions .

Is this even possible?

My tests with partial signatures don’t seem to have worked since trying to sign an already signed transaction doesn’t work.

Shelley command failed: transaction sign Error: tx.tmp: TextView type error: Expected: TxUnsignedShelley Actual: TxSignedShelley