Another thing that occurs to me as a potential problem with target switching: Attacks that constantly re-align themselves. Or, the actual movement patterns of certain things that do a similar thing (like the Warden).
This could produce some effects that outright look very glitchy. Heck, there's one boss that I'm fairly sure would outright stop working, creating a completely busted pattern that ends up pretty much accomplishing nothing.
The "Persistence" enemy is a good example of the sort of thing I mean. They use homing shots that work by re-aligning every .7 seconds or so relative to the player's position. If the thing is suddenly switching between two players, it's just going to act buggy and weird. There is no fix for that sort of thing. Or a recent addition, is an enemy that forms these lines that constantly stretch towards the player. With two possible targets, it's more likely to form a distorted and possibly undodgable wavy mess. The last thing I want is to give players the impression that the game is buggy/broken, simply because it doesn't know what to do with any of that.
As it is, the Warthog's turrets might already be an issue, just not one that's become apparent through testing. I should probably try testing that one myself. But if it is producing issues, that's nothing I can fix on my end of things.