Q. Can I use a single Virtual Network in Azure across two different subscriptions?
A. An Azure Virtual Network is bound within an Azure subscription. It is therefore not possible for multiple subscriptions to use the same Azure Virtual Network. The solution is to create separate Virtual Networks in each of the Azure subscriptions with each using a different IP address space. You would then connect the separate Azure Virtual Networks in the different subscriptions together using the Azure site-to-site gateway (or ExpressRoute) which can be used to connect virtual networks. It is important when using this approach to realize the various bandwidth limits associated with the Azure site-to-site VPN gateway as this will limit the bandwidth between the virtual networks. This approach is shown below.
Other options that could be used include virtual network gateway appliances that could run within Azure.