AbleNet is available since AbleSet 1.6.0. It allows you to control two or more computers in a redundant playback setup. AbleNet is decentralized, so any computer in the setup can crash or lose connection. As long as one computer is still online, AbleSet will continue working.
I've created a short video that explains the basic steps:
In short, to setup AbleNet, open the same Ableton session and enable AbleNet in the settings menu of AbleSet on all computers. The computers will automatically connect to each other and can now be controlled simultaneously with AbleSet.
You can open AbleSet in the browser or on your mobile device using either computer's IP address. When the computer you're currently connected to goes offline, AbleSet will automatically redirect you to the next available computer.
To connect two computers to the same output channels, you could use iConnectivity's PlayAUDIO12. This audio interface allows you to connect two computers and automatically switches to the next computer when it detects that the current one isn't outputting audio anymore.