SAP B1 doesnt support that. But i think u might do it this way to simulate,
1. Create a UDF on the document you want to have this 2nd sign on feature.
2. Create a UDT to hold user code and their password pair (remember to use md5 or something to hide the password)
3. Use SP TN to block those transaction without correctly entering the UDF field. (It should be the password)
The only problem might be the password is clearly saved, which will be against FDA rule though.
Frank