If you had a wireless router that would take care of the Internet connections for the two separate inputs (wired or wireless)...I know it's not what is desired but a second tower to send that input would be needed so you could both surf at the same time.
Is the use of a second tower a possibility at all??? I know used towers can be inexpensive at some of the local compter stores as they get traded in all the time, cleaned up, all they leave on them is a basic operating system. It may be less expensive in the long run than to have someone build you present system into two independent ones.
We're running a UBEE wireless router here that has all 4 connections being used. #1 is the Wifes computer, #2+#3 are my two systems here in the office(one of which is run split screen)...the first three are wired connections and #4 is being used by the Wii wireless.
One of my systems, the used single tower which was someones trade-in, cost $125 and it has an AMD Sempron 3,000+ 2.00GHz, 960MB RAM, running Windows XP, it has 2DVD CD-RW Drives, 4 removable disk drives, 75GB C Drive.
There are good deals on used systems out there if it's an option;)