I would suggest an Ethernet switch, however, if you are on the cheap and willing to try it, you can either make or purchase and Ethernet splitter. I have never tried one of these and they can possibly cause you problems, but for a somewhere under $10, it might work?
Ethernet Splitter
But for around the same amount of money you can get an Ethernet switch that will work and allow you to grow a little bit!
Ethernet Switch
If you really look around, you can use any wireless router for this purpose as well, do not connect the WAN port, disable DHCP and the wireless and just connect in on the LAN side. I have found some older Wireless B routers with 4 port switches for free after rebate!
JamesO