Welcome to Daran Headquarters!
What is Daran?

Daran is alternatively known as Randa/Randaza or Ranzai. There are two versions of their ship names because as we use Japanese fandom ship formatting etiquette, this inherently includes ship top/bottom preferences into the equation to help filter searches and find the specific posts you like and spares you from discourse. Contrary to popular stigma, Japanese fandom has always had a way to include ships that have no exclusive top/can switch. Repeating the Kanji of the initial first character a second time allows for both 'versions' of the ship to pop up in a search.
(Ex: 太乱太 — it includes both daran (top Dazai) as well as the kanji for ranzai (top Ranpo) when you search it, allowing for you to find all you are looking for, while those who only like one or the other can exclusively find the version they like!)
What about the name Souheki?

The ship of Dazai and Ranpo has long existed before the mention of this information from 55 Minutes, which came out 4 years after the series itself. (And changing ship names is sort of a no no, no matter the fandom!)
Then... What should I do?
While it is an accepted fact that all of the Soukoku names are equaled with the ship in western fandom, that is not the case for the E.Asian fandom. The art they tag as 双黒 will usually only ever be general, unromantic/nonsexual art of the two because team names are not interchangable with their ship names, and it further helps protects people looking exclusively for the content they want, as perhaps some people only like them as a buddy duo, sibling-esque, friends, etc!
This is the case with 'Souheki' as well. Souheki is the team and 'platonic' name for Dazai and Ranpo, which is why none of the E.Asian artists who have been creating for them for years have ever used or made much mention of the name because they like the two as a couple.
(However, the Ranzai artist Shirousagi DID make a Souheki/Platonic focused Doujinshi you can glance at here. So it's not as if it's totally ignored, but it's not for most and what they're aiming for in their fandom adventures!)

This website plans to dedicate to all variations of these two, but naturally will take priority with the romantic angle. Happy exploring of this ever growing site, dedicated to the two lil guys I love a lot! For more information on the use of Souheki as a term, feel free to click here.