I am looking for two words/names/terms that refer to something that as a whole cannot exist without each of the two. I am not looking for a term for the "whole" as this "whole" may not have a known name itself, as it is commonly defined by its parts. Each part is as important as the other. The two terms may have not an actual meaning, but if then it should be neutral or positively connoted.
The best I could think of was
Yang. Are there other suited terms?
Ideas I discarded are given below [+ argument against]
Alpha + Omega [implies beginning and end] First + Second [implies ranking] Mother + Father [implies gender] Good + Bad [implies good & bad] PartA + PartB [implies lack of imagination] Part + Tarp [implies failed attempt at being imaginative]