Spring still has the Dual Screen Mode option available. When the game isn't running, edit springsettings.cfg and add DualScreenMode = 1 somewhere. This currently doesn't work as well as it used to, and it still comes with all the caveats it did previously.
The option became bitrotten over the years, so rendering with the option enabled is going to be off until Zero-K moves to at least Spring version 104.0.1-1554-gf775ea1470.
The fix in question. Zero-K itself recently had its base widgets fixed to handle dual screen mode correctly, so there shouldn't be anything further to do on that side.
There are a lot of limitations that have always been present in Dual Screen Mode - for instance, it always assumes two monitors of equal size. So if you have two monitors of differing sizes, you'd probably want to either fix spring to handle general cases, with all the complexity that entails, or hack around it by (for example) having a "fake" monitor using xvfb that rescales and renders to the real monitor.