PDF supports multiple signatures on a single form or document. When using a form, you can use field collections to associate specific fields to a certain signature and lock those fields after signing, while leaving other fields accessible. There are many posts throughout this forum that discusss this functionality, here is one... http://forums.adobe.com/thread/769340
As for signature status, applying a second signature (or more) after a form or document has been signed will NOT invalidate the original signature, however it will change the status of the original signature to something like "Valid with signed changes". Each subsequent signature will change the status of the previous signature, but this is not the same as invalidating a signature.
This behaviour is by design as a signature applied to a PDF signs the entire byte range of the PDF, not just a portion of it.
Thanks for your quick response.
The signature doesn't get invalidate untill an unless any field in the pdf is modified. After signing the pdf i want to insert date in the pdf . i.e. it will modify the pdf. After the pdf is modified (date is inserted in the signed pdf), the signature gets invalidated (the red cross apprears on the signature field).