Media Center 15 introduced the ability to link two or more zones. These can be hardware zones such as you might have on a single PC by using multiple sound devices, or they can be DLNA zones, or any combination.
If your PC has more than your primary zone, they will show up under Playing Now. If there are multiple zones, your primary zone will show up under Playing Now with a name that is the same as the machine name.
To link two zones, just drag one onto the other. If anything is playing in either zone, both should begin to play it in a few seconds.
You can adjust the timing of the zones so that they are syncronized by right clicking on one.
Right clicking also provides other options such as renaming.