If your pc / laptop next to each xbox has both wireless and ethernet ports then you can use it to 'bridge' the wireless connection to a wired connection for your xboxes.
You can indeed use to xbox machines from the same address, I don't think you can sign on twice though with the same xbox live profile / account.
Regarding your wireless adapter, you don't have to do it like that.
The device on this link
http://www.ebuyer.com/UK/product/112497/rb/250 02329813
can allow you to use your wireless network but connect your xbox to it using a wired connection. They work out at �30 each but you may be able to get them cheaper.
Hope this helps.