Do you mean that you have two different brand network adapters in the PCs, one per PC? If so, then you are connecting to the same router and they are on the same local network.
Even if both computers were connecting to the Internet through different ISPs - say one was connected to a DSL high speed and the other were on dial-up using a modem and phone line, the fact that they are activated with the same LiveID for the same subscription makes them part of the same Circle.
-steve