Virtual networks that are created through the Azure Resource Manager deployment model and classic deployment model—but that belong to different subscriptions—can now be peered through this preview release. Read the Create a virtual network peering using Azure PowerShell article for further details.
Note that you can peer virtual networks that exist in two different subscriptions as long as a privileged user of both subscriptions authorizes the peering and the subscriptions are associated with the same Active Directory tenant. You can find more information in the Virtual network peering article.