To connect a node to already existing private network on a different VM, you have to setup Ethereum on the new VM and then connect. You have already setup Ethereum Private Network on one Ubuntu system (VM-1). Now, you must create an Ethereum Network on the new Ubuntu System (VM-2) using the same steps that you used to create the network on VM-1. While created the network on VM-2, make sure to satisfy the following conditions:
1. VM-2 must have the same Genesis Block as that of VM-1
2. The network for both the VMs must be NAT or Bridged network
3. Port numbers on VM-1 and VM-2 mentioned in the geth command to start the Blockchain should be different.
Once you set up the system to meet the above-mentioned conditions, you and add the peer. On VM-2, run the following command in the geth console to get the enode:
You will be displayed an enode URL.
Now in VM-1, in the geth console, run the following command by replacing the enode URL that you got on VM-2.
Now, the peer will be added.