This is not mentioned in your question (and the link assumes your OS is not in a VM), but perhaps you haven't configured VirtualBox to handle the network card properly.
This can be done by shutting down the VMs, then right-clicking each and choose Settings. Go down to Network and finally go to the Adapter 1 tab.
By default, VirtualBox sets the network adapter to use NAT, which (in layman's terms) means it gets a private IP and shares the host's network connection, but it doesn't have any information beyond that. You are probably looking to either use Bridge Adapter or Internal Network.
Using Bridged Adapter means it will reach outside to your router and act as a separate device on the network, as if it were a different physical box on your LAN. It will be able to get an IP address from your router/DHCP server.
Internal Network creates a virtual, internal network just for VirtualBox (based on the Name field). This configuration is a bit more secure, but it can be harder to configure because you won't have access outside of VirtualBox to the internet or your LAN. You also won't have a DHCP server set up, so you'll need to assign IPs manually. (It's outside the scope of this answer, but you can set up a 3rd VM with PF Sense as a router and give it BOTH a Bridged Adapter interface and an Internal Network, and it will act as a private router for your VMs. This is the most secure option.)
In either case, as long as you use the same method for both VMs, you will have IPs for them and they will be able to connect to each other.