I am a cable/internet tech and not 100% sure on your ADSL ..but If it is directly connected to the server I am guessing it is "Bound" to the Mac address of that machine, and wont work on another...if you have a router inline "first" then split to both machines you should be fine ..if your server is connected directly to the dsl modem and you are networked through a second ethernet card so you can share files etc. computer to compter then I am guessing also you may be able to "share" the servers internet connection if it is specified in the Network settings somewhere that ,that is what you want to do TCP/IP settings etc...sounds like though you may be in over your head there...best thing to do is contact your ISP and have them do it right for ya....
or screwing around you may knock your server off line...