Acrobat Pro DC won't allow more than one certified signature

Hi,

Hope you are doing well. 

Certificate signature has always been like this. Once applied by a user, then it can't be added further. 

For example, if you have added your certificate signature to the document, then it wont let you sign it again, because once the document is signed, its restricted to make further changes. 

What you are referring above, was probably a PDF form, assigned with multiple "Digital Signature" form fields. There, multiple signers can apply their certificate signatures on assigned Digital Signature filed. For more infor, please refer to this help article: https://helpx.adobe.com/acrobat/using/creating-distributing-pdf-forms.html#creating_and_distributing_pdf_forms 

Thanks,

Akanchha