Freebase Dali

If they live in the same house they're either using the same computer or can be using different computers. The IP address you see is the WAN IP address their ISP gives them and is only on the outside interface of their router, and is utilized via NAT by all computers connected to the router.
Their physical IP addresses would be in the 192.168.0.0 network. That's a private IP range and you can't route with it outside the LAN... so you can't really go by her/his WAN IP address to make any conclusions beyond the fact that both computers/users are connected to the same router.

(unless, of course, you see a local IP address as well as a WAN IP... do you? If so, this only tells you they may be using the same computer to log on, or he is in fact the same person. No way to know for sure.)