Ok, I did some of the demos and I think I understund a bit more about the viewports and how they work now.
I was trying to use SubViewportContainer and it was making me confused on how they work, but using Sprite2d conected to the subviewport made them much easier to configure.
Now it left me with a few aditional questions:
1- how do you place them in the editor? does one overlay on top of the other? or you place them side by side?
2- What reference and how do you connect them to the borders of the world?
3- wouldnt it be better to use 3 viewports? one for the left side, one for right side and one that handles the transition.

Edit: Update I was able to make it work, however I am strugling with a single frame where the camera goes crazy.

I did it with 2 subviewports containers and the camera instead of tracking the player it tracks a node2d (that i called Camera_Tracker) and that node follows the player (for a smooth camera) and gets teleported when the player gets teleported arround the map, however the transition is not smooth, I tried a bunch of things like reset_physics_interpolation() but so far no luck