What happens if one heir confirms the Mukatabah while the other denies it?

Primary text

If one heir confirms and the other denies, the Mukatabah is established for half of the slave, while the other half remains in slavery. If the confirming heir testifies on behalf of the slave against the denying brother, the testimony is accepted because he does not gain benefit nor ward off harm through it. If another witness joins him, the testimony is complete and the Mukatabah is established for the entire slave.