What is the difference between a single downpipe and those that have 2 going into one (sometimes a thinner one merging with the main portion)? And what is the reasoning between each layout?

 

Dull & complicated. Theory says one thing, practice says another. It's one of them.
Both work well - differences between them are minimal compared to the overall improvement of getting rid of the cat/blocked wastegate exit of the oem DP.
Get whichever is easier to get hold of/cheaper; as long as they're built properly, it's all 'shades of blue' (or whatever the expression is that says that there's no difference worth bothering about ). You can tell there's barely any difference 'cos of the number of arguments thrown about here...