Hi, I've made a simple homebrew for this purpose. SplitJoycons. It automatically change grip to horizontal mode allowing 2 players controls. Usage is very simple, just open the app, detach joycons and return to your game. It obviously requires access to homebrew menu.
Let’s don’t forget that SciresM was the author of Boot9strap on 3ds. Surely he will find a way to run Atmosphere at every boot without needing to going on RCM every time.
It's not a tool, it's a framework/physics engine in C++. Maybe we just need a more complete SDK. Also it'll be even easier if we could use Cocos2D. Maybe @dimok could explain me.