First, let's establish what "yin" and "yang" mean and describe the relationship between the two. According to Encyclopedia Britannica, "in Eastern thought, [yin and yang are] the two complementary forces that make up all aspects and phenomena of life" (Yinyang, Britannica).
To further the definition, yin is the "female" and yang is the "male" component of the duality. Yin is associated with stereotypical female traits like passivity, and yang with stereotypical male traits like assertiveness. The thought is that together yin and yang make a whole, and that they complement one another perfectly.
In The Joy Luck Club by Amy Tan, Rose and Ted could be described, at least for some of the novel, to represent yin and yang. Certainly, they conform to typical gender roles early on: Rose is much more passive in the marriage, while Ted is more forceful. They are described as being interested, when they meet in college, in opposite specializations, with Rose wanting to major in liberal arts (then fine arts) and Ted majoring in pre-med. Rose's Chinese heritage is important to her character, namely to her relationship to her mother. An-mei calls Ted simply "American" as a way to explain differences between the spouses, even though Rose grew up in California.
Once their relationship progresses, Rose observes that "we became inseparable, two halves creating the whole: yin and yang. I was victim to his hero. I was always falling and he was always rescuing me" (125). Ted, the man, of course gets to be the hero, the savior, the one acting in every scenario. Rose, the woman, is the passive victim, the maiden relying on her shining knight. Rose describes both of their dependency on this structure, which, at first, seems to benefit and satiate them both. Over time, however, their marriage falls apart, so the yin and yang cannot, at least not in this modern American marriage, keep them together. When the divorce is being negotiated, Rose asserts herself and demands to keep the house. Being used to her former passivity, Ted is surprised, but Rose learns that asserting her own desires is more important than being the secondary half in her yin-yang marriage.