I notice here that you've had no replies - did you ever get this resolved to your satisfaction? I'm in a similar situation, I believe, where I need to provide advice on a workflow and form that requires two signatures, one from the employee and another from a supervisor. After the second signature, it gets sent to a clerk for processing.
You can set "seed" values in the signature field. A seed value can control the behaviour of the field. In the case mentioned in the original post, you coul set the seed value of the "supervisor" signature field so that only the specific digital certificate owned\held by the supervisor could be used to sign the field.
You can set signature field seed values so that the signature field can only be signed by...
a specific digital certificate (Signing Certificate)
any certificate issued by a specific certificat authority (Issuers and Policies)
When you are designing the form, select the signature field and click the "Settings" button in the object palette for the signature to open the seed value configuration dialog.