If two people are working from the same IP address, don't they each need their own verified PayPal account. This way you can tell if the person is legitimate or they are trying to scam the system using two accounts. I've seen this in so many sites that more than one person from the same IP address joins a site under a different account. They do this to help one of the accounts earn more money.
Here the site is paying for upvotes. If one account answers a large number of question each day, the second account could easily come back in and upvote the work so the first account can earn extra money.
If the site allows this to happen then more and more people will start to game the system on here. The only way I can see this really working is if the two people are willing to send you their ID and prove they have their own PayPal account that is verified through PayPal. This will prove that there are two different people working on this site. Otherwise, one person can upvote and only ask questions, the other can answer and make all the money.