Multiple Signers on Web Form

I need to setup a web form that allows multiple signers without knowing their email address before hand. In this case, the first signer is an applicant and the second signer is the applicant's witness.

I've seen videos online that show a form where the first signer enters the email address of the second signer. However, when I setup multiple signers on my web form it requires that I put in the address of the second signer. However, that address will change with each application.

Can this process be achieved?