My suggestion for this would be to set up a SSH connection between the two machines and then allow X11forwarding on a per user basis so that traffic occurs through an encrypted tunnel.
For convenience, I set up SSH using key authentication without passwords. For security, I disable SSH root logins. There are numerous guides on the internet on how to do this.
X11forwarding on a per user basis is enabled by adding to /etc/ssh/sshd_config
Code:
Match User <username>
X11Forwarding yes
Match
Please note this technique does not allow for a full desktop session to be forwarded. You can only use single applications, although if you use a file manager you can launch other programs from within it (e.g. open a shell, view a pdf document with okular).
The problem with using Xforwarding directly comes from the possibility of another user attaching to your display and being able to see what is on your screen. Network traffic is sent in clear and so could be captured and easily read. There is also potential for leakage of information by looking at the output of ps.
