That's really disappointing.
I only kickstarted the sequel because of how much I loved that I could play splitscreen with my other half.
During the kickstarter Larian had said:
"Two very different user interfaces will be supported - one for mouse & keyboard and another for controllers. If you play with controllers, you'll be able to use the same split-screen feature introduced in Divinity: Original Sin - Enhanced Edition."
I hope I've misunderstood something, because this feels like a big step backwards.