Here are some thoughts::
Check to see if uPnP is enabled on your router. If not enable it.
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Try setting manual/static IPs for each PS from within the router. Usually this is done via their MAC address.
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If it still doesn't work, you can try disabling uPnP and forwarding the ports manually for each IP/MAC.
http://portforward.com/english/applications/port_forwarding/PlayStation_Network/
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Another thing you could try without manually forwarding the ports, leave uPnP on, put one of the PS's in DMZ on the router.
