The problem is that both Xbox need a separate IP address from your ISP, you cannot nat Live connections.
The easiest might be to do the following, this would be my suggestion.
Assumptions: Both Telus and Shaw give you 2 dhcp addresses.
Set your Xbox with DHCP.
Connect your modem to a dumb hub or switch, connect both xbox to the switch, and connect the router to the switch as well (this allows you to isolate your computers when you want to game head to head via Live). When you want to have both Xbox on Live, you will need to either unplug or turn off your router.
The problem is that that Xbox Live needs a non natted IP address. Meaning that if you have 2 xbox and 1+ computer or other devices plugged into your router, your router doesnt give your xbox an un natted IP unless you port forward (this only works with one xbox, as you cannot do this with 2).
So, when 3+ deivces are plugged into the router only 1 xbox will likely work with live, while the other ip address (home accounts normally are given 2 ip addresses) is taken by your computer.
Your problem on the web from our dark overlords at google.
http://forums.xbox-scene.com/index.php?showtopic=338065