As per the description above, you want to select multiple form fields for signature, is that correct?
Please refer to the following KB docs which discuss the similar scenario How do I input multiple electronic signature fields to a fillable form? Link: https://forums.adobe.com/thread/2286908
Let us know how it goes.
Thank you for reporting the issue. Request you to please upgrade to the latest version of Acrobat since we are no longer supporting Acrobat X.
Please go to Work with Certificates Option and once you open a pdf with multiple signatures fields.A panel on the left hand will open up that will show you all unsigned signature fields