Media Center 15 introduced the ability to link two or more zones. These can be hardware zones such as you might have by using multiple sound devices on a single PC, or they can be zones from other copies of Media Center 15, running on your home network (requires enabling DLNA in Media Center's Options > Media Network on both machines), or any combination of the two.
DLNA devices are challenging to link since they can not be controlled with the same precision as a Media Center zone. As such, linking of DLNA device zones may not work reliably, depending on the hardware. This may improve with time as DLNA hardware improves, and J. River is able to test and tune for more DLNA hardware.
If your PC has more than just your primary zone, the zones will appear under Playing Now. When there are multiple zones, your primary zone will show up under Playing Now with a name that is the same as the machine name. In previous versions, this was Zone 1.
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. Anything you choose to play in either zone will now play in both. More than two zones can be linked.
You can adjust the timing of the zones so that they are synchronized by right clicking on one.
Right clicking also provides other options such as renaming.
Here's a thread on Interact that discusses zones.
Link Timing allows you to speed up or slow down the intra track playback for any zone.