If they work together and are posting from work then you would expect them to have the same IP address. They might also share the same IP address if they were in the same apartment building or (as has been said) go to the same school.
The problem isn't really that they might be the same person (though it is dishonest). The problem is that they might act in a collusionary manner. You might have the same problem with two people in the same family, or just two random people. Make sure you have rules to handle situations like that and you should be okay.
Having said that, I would do a traceroute on the IP address and at least see if they are honest about their location. Here's a good one:http://whatismyipaddress.com/staticpage