I have a travel expense document that once it is signed, it no longer allows the second signature field to be signed. When you attempt to add an additional signature, the field does not respond to button presses. Here is a link to the document on my Skydrive: http://sdrv.ms/OPi2Te
I am using Adobe X. I am not sure what version the document was created with.
The signature fields are configured up to lock all of the fields in the document when signed, which includes the other signature field.