Dual notary-signing

Hey there,

new to Corda, can some one help with explaining how the dual notary signing mechanism works? I am a bit confused about the locking and unlocking mechanism.

So if a corresponding transaction B is locked with the hash of unsigned (and un-notarised) transaction A proposal, how would the hash of the signed & notarised transaction A unlock that lock? Is it just that the notary or sender signature is not part of the txn has, but only validating requirement?

Can any one point me to an example of the locking and unlocking scripts?

Thanks