Dogz and Bluemax3,
Flatfour pretty much summed it up. It should be noted, however, that there are y-harnesses that have built-in reversers for one side, so that the servos will travel in opposite directions. That's what I have on my DP Extra, and it works well. However, if the servos don't center perfectly with each other, you may end up with more travel on one elevator than the other. This is where multichannel mixing comes in.

For example, in Futaba radios, channel 2 normally controls the elevator servo. But the 8 and 9 channel radios use channel 8 as the second elevator channel, if you decide to activate that function. So basically you have channel 2 controlling the left elevator and channel 8 controlling the right elevator, and both are activated using the elevator stick. The advantage to this is you can fine-tune the centering AND travel for each elevator half. Don't worry though, it's nothing you'll need to concern yourself with until you start flying larger planes with split elevators. I flew a standard 4-channel radio for several years without needing (or wanting) to upgrade.