@reasercher101 After the response is completed by one email the retake link of that response via workflow to second person to fill the same survey. 

Hope it helps!

Try this: if you are using contact list or directory so you can create nested authenticator. After first person consent, it will show another authenticator and will ask next person to enter his ID and then both will give their consent then will proceed for survey. I haven’t tried this but I’m sure this will work.