Do you mean digital signatures based on a signing certificate? If this is the case then you confuse terminology. What you mean is Digital ID aka signing certificate. You are in the right dialog. Click on "More,,," next to "Identities&Trusted Certificates" and you add as many Digital IDs (signing certificates) as you wish. The "Sign" dialog will allow you to select whichever certificate you want for each signing.
If you mean e-signatures, then you are in the wrong dialog. You want to use Fill&Sign tool for that, and it allows you to save only one signature, which you can later change, but not store several signatures.