Well that's pretty bad.
Personally, i think if they don't want to splitscreen combat (which makes sense), they should duplicate the exact same combat screen to each monitor. That mergescreen is unusable if the monitors aren't side by side, and even then it puts a great big black bar in between the skill icons.
Its not a viable option, because no matter how small your monitor borders may be, the cursor is split in half due to being in the middle of both screens and the camera is fixed onto that cursor, like in shooters. Right now I am forced to use one screen because of this. Which really is awful, since the entire HUD elements are created for TVs (huge) and not for small PC monitors. When I use splitscreen on just one monitor, it gets even worse.
The circumstance though, that Divinity alread has working dual monitor splitscreeen AND, even in combat, when one player opens the game menu, the screen gets split, I really see no reason at all not to offer that option.