It should be possible to use two Xbox's through Live! at the same time, much the same as you can play on two PC's from the same IP address in most online games. The problem is probably the wireless connection being throttled on the 24Mbps line. The speed shouldn't make a difference, but if there are other applications using the internet at the same time (that you don't know of) then when you try and send a large amount of data through to the Xbox and vice-versa, you'll most likely experience a disconnect because the bandwidth has all been used and that's why the connection may be timing out.
I'm not exactly clear on how your friends Xbox makes its connection to the wireless router. Is it through another PC or is it through an Xbox Wireless adapter? Try altering the position of the aerials on each of the wireless devices. I find pointing the tips in the direction of the wireless signal to be a good way of getting a clean link. Hopefully this should get you more juice out of the connection.
|Thomas|, what kind of connection issues where you experiencing? When you manually configured the connections, did you put in the IP, Default Gateway, and Primary DNS? Your Default Gateway and Primary DNS should always be the IP address of your Router (if that is connected directly to the broadband connection). I've got these settings configured on each of my adapters in my PC first, and then I create a bridge and give the bridge new details. So for eg my setup is like this:
Wireless adapter IP: 192.168.0.7
Wired adapter IP: 192.168.0.8
Network bridge IP: 192.168.0.1
All of those IP's are allowed through the firewall in both the router and my software firewall using a security rule. When the network bridge has been created, I have to run the "netsh bridge set a 1 e" command before I am able to browse the Internet on my PC, as this has the wireless card inside it, which is in turn connected to the router, which is connected to the internet. That command forces certain wireless adapters to behave in a certain way, allowing you to browse the internet if you have connection troubles beforehand.
I honestly can't see that having a static or a dynamic IP address would make any difference, unless you didn't input the Default Gateway and Primary DNS IP's when you set the connection details.
Let me know how you get on.