Automatically do a two-way sync of two directories

Unison

Unison is a GUI and terminal-based tool which allows files and directories to be kept in sync with each other, between different local directories and drives or on a network, which could be on different operating systems. The application is available for Unix operating systems (Linux and Mac OS X) and Windows. Changes can be made on the different places, and Unison will update the machines with the correct versions of files and folders, copying, deleting, renaming or deleting files and directories as necessary.

Unison sync app is probably the most used and trusted by the Ubuntu/Debian community. It is available at the Software Center and at the package manager. It has a command-line and a graphic user interface (GUI)(GTK).

Unison Homepage

Ubuntu Help Community Wiki - Unison

Ubuntu Manual - Unison

A short use example, close to your request, can be found at Rmano answer (screenshot below).

FreeFileSync

FreeFileSync is a free data backup software that helps you synchronize files and synchronize folders. It is designed to save your time setting up and running data backups while having nice visual feedback along the way. FreeFileSync is Open Source software, available for Windows, Linux and macOS.

I've never tried but it seems to be worth a shot. It has a default GUI and is under active development.

FreeFileSync Homepage

Synkron

Synkron is an application that helps you keep your files and folders always updated. You can easily sync your documents, music or pictures to have their latest versions everywhere. Synkron provides an easy-to-use interface and has lot of features. Moreover, it is free, open-source and cross-platform.

Synkron Homepage

For a long time I used rsync to sync my laptops(2) and a 3rd machines, then I tried with syncthing but this morning a famous search engine offered me this project :

Mirror

You will also find a comparison of different solutions.

His main argument against rsync are : not real time and nor officially two ways.

ps: I'm using it through zerotier so my machine are always able to communicate together.