I had a similar problem. Not the same, just similar. I went through all kinds of hoops trying to get 2 SME servers running off of the same cable modem. It works just fine on Windows machines, or one SME and one Windows.... But two SME machines took persistence to get going.
I cannot explain why it did not work, nor can I explain why it now works. I was getting my IP with DHCP send ethernet address and all was working fine. When I tried to setup the second machine, it would steal the first machine's IP. Every time...
I finally set the second machine to get its IP via DHCP send account name (and I used C-123456) and it worked.
So the morale of the story is;
1) keep trying
2) try sending ethernet address instead of account name
GOOD LUCK