Splitscreen - LambdAurora/LambdaControls GitHub Wiki
Since LambdaControls v1.2.0 the virtual mouse option has been added.
This means that 2 Minecraft clients can be used at the same time without having to pause on when interacting with the inventory, etc.
Steps:
In the launcher
- Create a first profile with Fabric and install LambdaControls.
 - Create a second profile with Fabric but change the Minecraft directory and install LambdaControls in that directory aswell.
 
First controller client
- Start the first client and go into the LambdaControls options.
 - Set input mode to controller.
 - Choose the correct controller in the list.
 - Enable unfocused input.
 - Enable virtual mouse.
 - In-game press 
F3+Pto disable pause on lost focus. - Configure everything else as you like.
 - Test.
 
Second client
- Start the second client.
 - If you want to use a controller on this client:
- Go into the LambdaControls options.
 - Set input mode to controller.
 - Choose another controller in the list, it must not be the same one as the first client!
 - (Optional) Enable unfocused input.
 - (Optional) Enable virtual mouse.
 - (Option recommended) In-game press 
F3+Pto disable pause on lost focus. - Configure everything else as you like.
 
 
After
Everything should work, now play!