I don't know exactly about ICS, but why would you want to do something like that anyway since you have 2 IPs.
Just get a hub or a switch and then you don't have to deal with any software solutions.
You can get a Linksys 5-port switch for around $30 from staples.
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I say $30 cause, unless there is a new rebate, the old $20 expired.
Since you both have a different IP, you should have no problems playing on any external servers at the same time.
And you will both be sharing the bandwith. But like I said before, you shouldn't notcice anything different than if you were playing alone.