Merge two domains

A client we manage had two physical Server 2003 machines that were physically and logically separated; one server was in building A and the other in building B on separate subnets.  The buildings were connected by a fiber line running between the switches in each building.  I recently upgraded one of the machines to 2012 R2 and virtualized it (ESXi 5.5).  I'm wondering what the best way to merge two different domains is?  They are obviously different forests too, but I was thinking of virtualizing the server in building B on to the same host as the server I already upgraded building A's server on and just dedicating one of the NIC ports to this new server.  All I'd have to do is unplug a few connections and re-plug a few in each building to get this to work.  Does anyone else have any other thoughts or ideas that might be smoother/easier?  

Thanks in advance.