Didn't want to start a new thread, I have the same problem as the OP.
Both macs are running Tiger, on the same home network behind a router (so same external IP ?).
When I try to log in with the same user name, the signed in mac will log out to allow the other to log in. This didn't happen in Panther as I was able to log in with both macs on exactly the same network setup simultaneously.
Anyone else (not) having this problem and can offer any help? I'm not interested in whethe ror not the pop-up appears, but just whether it's possible to log in simultaneously on this network setup. Is it a bug (affects some users) or a 'feature' (affects all users)?
Thanks