You can use show hide conditions to show and hide fields. Signature fields though are special in that the condition cannot result in there not being any signature fields. You can try and show an optional signature field though. Then again the check itself may be enough indication of intend along with the main signature.
Fields can be assigned to anyone, but as soon as they are filled out, they are locked for any next signer. or in other word signers cannot overwrite each other fields or collaborate within the same field. Each participant needs to be assigned their own fields.