using second ethernet port to share net connection

It' very easy... I'm assuming that you are using Leopard
FIRST:
Go to Sharing in the System Preference and set the internet sharing From and to settings and make sure you turn it ON

Second:
go to the network where there's internet...
* write down the IP, Subnet mask...

Third:
Go to the other network you want to share to...
in configure choose manually then put this IP address 10.0.0.1

NOTE: if the first network address has the same 10.0.0.X then put 192.168.1.1

Forth and last:
Go to the Macbook Pro network preference and set configure to manually then put these setting:

in IP: 10.0.0.2
in Subnet Mask: Subnet Mask you wrote down
in Router: 10.0.0.1

That's it...
I hope it works (^_^)