You're describing Hextech synergy, not an item. And it's pretty much on level with Phantom Synergy given its temporary and you can position around it.
Yes you have to pick between Assassins or Hextech for your positioning answers, but if you're scouting the players there should be a higher quantity of one or the other. That and its never going to hit your whole team unless they draft 4 Hextech, which would be an insane thing to do given that none of them have overlapping synergies. Jinx and Vi support Brawler and Gunslinger with just an extra unit, but you're going to suffer not having buffs for Jayce/Camille and having them just take up slots that you could bench them over.
If you're top 4 vs Hextech you definitely should be positioning your core carry to survive 7s and have several items on different champs. At that point you easily have 7 units if not more, you should have about 5 complete items at least too. You can still have a hypercarry with 3 items and sprinkle your other items on units one tile away, if you're worried about assassins you should have a junk tank like Warwick or Garen sitting next to your hypercarry anyway and that can create the space needed to avoid Hextech too.
Its an annoying bullshit synergy, but its not unbeatable. Hell given all the broken shit they added to the game with the Brawler's Glove we need Hextech to remain as is or the game will just turn into Assassin Tactics and stay there forever.