Having actually had two managers (and then, later, three) at one point in my career, I can tell you firsthand that it's a nightmare.

Manager A: "Drop everything and fix widget #123."

[later]

Manager B: "Why haven't you finished fixing widget #456?"

Me: "Manager A told me to stop working on that!"

Manager B: "Forget what he said! Work on #456!"

[later]

Manager A: "Why the **** haven't you fixed #123?!?!"

... etc. Oh, the two managers should have talked to one another and synchronized? Sure, they should have, but human nature works in different ways and people tend to disagree, especially when they're put into positions of what is supposed to be equal power. I don't, personally, believe that any form of matrix management can work in any organization where something is actually supposed to get accomplished.

But, YMMV. And apologies for the aside to the conversation 😀