I have never done this on purpose, but it's happened to me at least two separate times.
1. two patients who independently become my patients over the course of a year. One of them said "I'm going to my girlfriend's parents' house for the first time" and a few visits later the other of the couple said "my boyfriend came to my parents' house for the first time. Oh, right, I should tell you..." I see them both for med management, but just the other day one of them was accusing the other of being agitated and verbally abusive, and the other was having less depression and less brain fog and more paranoia, so I switched his meds around. He had not been open with me about the psychotic symptoms in recent visits, so I'm very appreciative that his spouse said something to me. I hope it wasn't too big a rupture.
2. two patients independently become my patients within the same month. I treated them without knowing they were a couple for at least a year. One day the first one says "I broke up with my girlfriend, it was amicable and mutual." The other says "so my boyfriend just broke up with me, it was very sudden and one sided. You know him, he's [name]. I know I should have told you, but we actually started dating after we both became your patients!" They were nice enough, and he was moving abroad after that visit, so she continued in treatment.