Ableton offers a great feature that allows you to sync instances running on different computers to each other using MIDI. One instance acts as the master and all other instances follow its playback. You can use this feature to control multiple Ableton instances with AbleSet.
The setup is fairly simple. First, connect your two or more computers via MIDI. On the master computer, go to Ableton's preferences and enable the Sync option for your MIDI output:
On the other computers, enable the Sync option for your MIDI input from the master:
Lastly, enable the Ext button in the top left corner of Ableton on those computers as well:
And that's it. Your Ableton instances will now play in sync. To control all instances using AbleSet, simply install it on the computer where the master instance of Ableton is configured.
If you don't want to use a MIDI cable, you can also transmit the MIDI signal over ethernet or WiFi. The YouTube channel From Studio to Stage has a video explaining how to do it on macOS:
On Windows, you can use rtpMIDI to get the same functionality. It is also compatible with macOS' protocol so you can connect Macs and Windows PCs as well.