Making your reports 2x as productive is far more expansive (and less visible) than just ensuring they're working on stuff 2x as important or doing 2x as much work.
Think of software for a moment. Has there every been a library, language, framework, etc that you absolutely love to use? The API is just right, the framework is the perfect level of opinionated vs. not opinionated, it just gets out of your way and lets you accomplish what you want, it's consistently reliable, it has fantastic backwards compatibility, it's error handling semantics are clear and the right level of verboseness, the internals are so well architected that it's also incredibly performant, etc.
It's just such a pleasure to work with. It's always just what you need, right when you need it, exactly how you needed it.
A company is a system, just like software. And a team is a submodule within that system. A 2x manager is the person that is able to make (and maintains) their team into the type of beloved system I mentioned above. How they do that is dependent on their expertise at comprehending, navigating, and influencing the larger system, and shaping their team to be precisely what that larger environment needs.
As hard as it is to do the above, it's really, really easy for a 0.1x manager to destroy that type of system/team. A few breaking API changes ("my team no longer handles those types of request you've been giving my team for years" vs "I've spoken with team x and it turns out they're more appropriately staffed to complete that work. Don't worry though, I've got a change management process in place and will ensure the handoff is seamless for you", refactoring some internal code that kills performance (i.e. losing important ICs without any risk mitigation/change management in place), etc can all tank the goodwill and community adoption of that once beloved system.
I switch between being an IC, a purely people manager, a functional manager (i.e. a team lead with people management and hiring/firing authority), and a "manager" (that's effectively an IC with a manager title so I have the organizational authority to execute the type of work I was doing). If you've never been a manager before, let me tell you: doing that shit right is exhausting. The biggest difference between being an IC and being a manager is that, as an IC, I'm able to ignore 90% of the bullshit around me and focus on my work. As a manager (whether doing IC work or purely people-managing), I can't ever tune out that organizational noise/bullshit/dysfunction/needs, because the more in tune I am with that the more likely that I can position my team to support the company, and in doing so ensure the peace of my team.
It's hard to image what a 2x manager is, because there's no single way to define one. But as an IC, having (or losing) a 2x manager will have a noticeable impact on you and your team's productivity over time, in any of a dozen or so different ways.