With an Ethernet "splitter", you can simultaneously connect two computers (or other network devices) on one Ethernet cable. You can buy Ethernet splitters for approximately $ 20.00 USD but you also can make your own.
One office, one ethernet jack and two computers, or
One living room, one ethernet jack and one HTPC and one XBox.
If you can't realistically (without tearing apart walls or renting a scissor lift) pull one more ethernet cable from the patch panel to the office / living room etc. you can consider the use of an Ethernet "splitter".
I'm assuming all the four pairs of the ethernet cables are properly connected within the ethernet wallplate and the patch panel.
Step 1: What you need
In order do make your own Ethernet splitters you'll need the following:
Step 2: Crimp the RJ45 Plug to the ethernet cable
...using the following wiring scheme:
Taken from the excellent Hardware Book: www.hardwarebook.net/cable/network/ethernet10basetstraightthru.html
Step 3: Punch down the other end to the RJ-45 keystone jacks
Take the other end of the cable, cut it to 9 inches and punch down the four pairs using the following wiring scheme:
Step 4: Make one more splitter
...using the previous information so you end up having two splitters.
Step 5: Plug one splitter to the RJ45 wallplate
... and plug the two computers to the splitter.
Step 6: Plug the other splitter to the patch panel
...and plug two patch cords from the splitter to two free ethernet jacks from the switch. If your splitter connections are right the two leds "LINK" from the switch on which the patch cord are connected should turn on.
Step 7: Final wordsWe now have two computers simultaneously connected using only one ethernet cable. This method is similar to PoE (Power over Ethernet) but instead of injecting DC, it is injecting another "data".
But again, it is better if you can to pull one more ethernet cable from the patch panel to the wallplate. But in some specific cases, the Ethernet "splitter" can help out.