I've not really noticed it, however I have seen:
A day where the neighborhood had a lot of "win a raid for a rare candy" quests (6 within a 20 min walk).
A day where the "evolve two Shellder for a Lapras" quest was clustered about 6 of them within a 5 minute walk of eachother.
Otherwise I see similar quests together, but they're common quests so I write it off as luck. Like two "power up a Pokemon 3 times" close to eachother doesn't really surprise me. I guess it happens pretty regularly though.
I have never seen Chansey quests beside eachother though. In fact, I haven't seen more than 1 Chansey quest in the same larger neighborhood in the same day (have seen one reported downtown and one reported in North Toronto, about 15km apart).