You can do host-only networking and connect guests to other guests on host-only networks on multiple hosts, or multiple host-only networks on the same host. This assumes a little bit of network configuration knowledge, and some familiarity with VirtualBox (and at least a couple of hosts and guests.). Much of this text is the same, but some of the details have been made more specific to a generic case, and less about the specific case outlined in the forum question. I answered the question on the link above, but thought to put some more detail in this discussion. The longer and better answer is "yes, they can!" The general and short answer is "no, they can't."
Then, looking for something else, I stumbled upon a related question regarding VirtualBox Host-only networks, and allowing different host-only networks to see each other. A few days ago I wrote a bit about VirtualBox 4.3 NAT Networks.