howto set communications between virtual box guests

The Simplest and shortest way: 

1. Open the machine's setting.
2. Choose Network.
3. Set the adapter detail's "to "bridged networking" or  in a more updated version of Virtual box,     Set Adapter "Attached To:"  to "Bridged Adapter". 
4. Configure the guest machine with a permanent ip address according to your network's structure and the machine's operating system. 
5. Repeat 1-4 for the other guest. 

That it is. 

[updated: 04/09/2015]