Splitting is a type of cognitive distortion where one sees a situation from an all-or-nothing perspective. What could we call the type of thinking that is not based on extremes? Would "balanced thinking" work?
Why not simply "thinking clearly"?
The very activity of thinking itself is already free of such biases. The biases arise from influences external (such as prejudiced beliefs, identification, etc.) to the thinking process itself. When you think clearly, you are said to think without letting those influences taint the thinking process.
Another term for "splitting" is "dichotomous thinking". A straightforward antonym of this would be "non-dichotomous thinking". This isn't a super common term, but it does turn up plenty of relevant results in Google, and it's quite clear on inspection what it means.