Live tile content (tile updates) can be sent to secondary tiles in much the same way that live tile content is sent to primary tiles. The secondary tile identifier needs to be specified when creating the TileUpdater (or the push notification channel, if using Windows Push Notifications).
TileUpdateManager.CreateTileUpdaterForSecondaryTile(secondaryTileId)
MSDN has a quickstart on sending tile updates which applies to both primary and secondary tiles.
This behavior is supported in both Windows 8 and Windows 8.1.