If it's not obvious from the previous video, I have actually added support for multiple viewports
This is a modified version of the older 'abreed' demo, with two views on the same game world. The sprites seen in both views are actually the same entities.
The parallax effect in the previous video in fact uses 4 different viewports stacked on top of each other to perform the decoupled scrolling.
There is a bit more to this than meets the eye - each viewport is defined by an 'arena', which is made from a set of playfields (framebuffers), each of which is updated independently as a distinct game view. You can actually load different maps (of different sizes!) and tilesets into each viewport if you like.
The same sprites/entities however are rendered into all viewports. I may upgrade it in future to maintain more than one game state, so a different game can run in each viewport. Because... well because we can.
The parallax demo actually uses a slightly stretched (7:6) version of the main map & tiles in the 4th viewport, to make the parallax work better.
The screen splits are implemented with a kind of TimerB copperlist/chain. Each arena can specify which screen split it is bound to on the hardware side. This part is not openly programmable yet, but I will probably allow the split configuration to be specified from the game code soon, and updated at runtime.