I am working on partially signed bitcoin transactions (PSBT) and have a question about the role of secondary signers in the signing process. I understand that in a multi-signature scenario, each participant must sign the transaction. However, I am wondering if it is possible for the second (or subsequent) signer to add additional data to the transaction in addition to the signature.
Here’s what I want to know specifically:
Can a second signer add additional metadata or input/output? Does the PSBT format support modifications by the signer other than adding a signature? Any insight or references to the documentation would be greatly appreciated!