Allow me.
I did a few searches and the info is there, you just need to look harder. I would call them split dumps but a lot of people call them twin dumps. There's your first point of confusion.
Your exhaust comes out of your turbo and also out of your waste gate. These exit points are a couple of cm from each other and if you have a single pipe dump they are combined right there at that point.
If you use a split you have one pipe for each which join together about 20-40 cms down.
The difference is that with a single pipe, when your waste gate opens, meaning that both exits are being used, there is turbulence created in the pipe thus slowing it down. You want smooth flow.
With a split, they join further down which creates much less turbulence and a smoother flow. the smoother the flow the faster the exhaust gases can escape, hence more power as the engine is less restricted.
How am I doing guys?