Yes, you need an ethernet switch. I hooked this up over a freind's house (not with a 360, but with a Xbox and a PS2). Basically, if I am understanding you correctly, you want to know if you can use an ethernet switch with the WPA54G Gaming adapter because the adapter only supports one console.
Yes, you can. A switch is exactly what you need to do this. The WPA54G is a bridge. It connects to a router and access point and behaves as a "wireless wire".
Get everything setup using the WPA54G attached to the Xbox and using the Xbox Network setup in the dashboard, test to see if it works, then shut down the Xbox, connect the consoles to the switch and the switch to the WPA54G and fire everything up.
You wouldn't want to connect to Live at the same time on both, but the beauty of this setup is it allows you to play System link without communicating over wireless, it uses the wired portion of the network for system LinK Play.