I tested this with a form created with LiveCycle Designer ES, and signed it with Acrobat Pro. I was unable to duplicate the behaviour, the remaining signature was not invalidated when clearing the other signature field.
Can you post the from you are testing with along with the list of steps you take to that generate the behaviour you are seeing?
Also, what design tool did you use to create the form?
If you post the signed file (use the Browse button at the bottom of the reply) I'll see if I can figure out why Acrobat thinks the signature is invalid.
I tested the form you posted. Here is what I found, but I am not completely clear on what all your script is supposed to be doing.
1) I filled in the first "comment line"
2) I signed the first signature field
3) Results: The signature was valid, the document status (see screen shot 1signature_docstatus.gif) reported the signature was valid. The signature staus was valid as displayed in the signature panel (see screen shot 1signature_detail.gif)
4) I added and filled in a second "comment line"
5) I signed the second signature field
6) Results: The first signature was valid, the second signature was valid, and the document status repored that ther were "unsigned changes" since the last signature was applied (see screen shot 2signature_docstatus.gif) The signature statuses were valid with subsequent changes made to the document as displayed in the signature panel (see screen shot 2signature_detail.gif)
Don't confuse the "exclamation" mark icon with "Invalid", it is only a warning to inform you that changes have been made that are not "digitally signed", I suspect your some part of your script is causing this. If you want to have Acrobat\Reader display a green check mark icon, then you cannot make any changes to the document after the final signature has been applied.
If you haven't done so, take a look at the sample I posted on your duplicate post in this forum ( http://forums.adobe.com/thread/492773?tstart=0 ).
If a signature is "invalid" it will display a red "X" icon.