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