I have mentioned this before, and I think it is a great idea.

Create an E-Mail Invite template in Survey Two.

At the end of Survey One, instead of having the system send an e-mail to somebody, have the system use information gathered in Survey One to fill in the template information that is needed for the e-mail invate created in Survey Two to send an e-mail invite.

What is great about e-mail invites is they are trackable and can’t be spoofed by people trying to stuff a survey.

I know your e-mail invites are designed for bulk e-mails and the above method would send out e-mail invites one at a time instead. But it would require a valid e-mail address to be entered and it would stop vote stuffing.