Some games will not work correctly based upon the type of NAT your router uses. I know that I had serious problems with The Darkness as the multiplayer would not work with type 3 NAT.
NAT Type 1 - Directly connected to the Internet ... no NAT or connected in DMZ.
NAT Type 2 - NAT with dynamic port allocation (UPNP)
NAT Type 3 - Strict NAT with no UPNP
With type 3, it may not be possible to communicate with other PS3 systems, and voice / video chat and the communication features of games may be restricted.
It sounds to me that you have a router that doesnt support UPNP, and that the 2 PS3s are having problems with resource contention. Your friends could try logging into the admin page of the router and checking to see if they can enable UPNP. Depending on how old the router is, they might be able to update the router firmware to enable UPnP support. If not, well the available options are few, and increasingly complex.
Bethesda's Todd Howard "if install base really mattered, we'd all make board games, because there are a lot of tables."
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